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‎README.md

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| # | Title | Solution | Difficulty |
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| :-: | - | - | :-: |
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| <span id="1354">1354</span> | [Construct Target Array With Multiple Sums](https://leetcode.com/problems/construct-target-array-with-multiple-sums "多次求和构造目标数组") | [Go](problems/construct-target-array-with-multiple-sums) | Hard |
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| <span id="1353">1353</span> | [Maximum Number of Events That Can Be Attended](https://leetcode.com/problems/maximum-number-of-events-that-can-be-attended "最多可以参加的会议数目") | [Go](problems/maximum-number-of-events-that-can-be-attended) | Medium |
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| <span id="1352">1352</span> | [Product of the Last K Numbers](https://leetcode.com/problems/product-of-the-last-k-numbers "最后 K 个数的乘积") | [Go](problems/product-of-the-last-k-numbers) | Medium |
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| <span id="1351">1351</span> | [Count Negative Numbers in a Sorted Matrix](https://leetcode.com/problems/count-negative-numbers-in-a-sorted-matrix "统计有序矩阵中的负数") | [Go](problems/count-negative-numbers-in-a-sorted-matrix) | Easy |
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| <span id="1350">1350</span> | [Students With Invalid Departments](https://leetcode.com/problems/students-with-invalid-departments) 🔒 | [MySQL](problems/students-with-invalid-departments) | Easy |
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| <span id="1349">1349</span> | [Maximum Students Taking Exam](https://leetcode.com/problems/maximum-students-taking-exam "参加考试的最大学生数") | [Go](problems/maximum-students-taking-exam) | Hard |
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| <span id="1348">1348</span> | [Tweet Counts Per Frequency](https://leetcode.com/problems/tweet-counts-per-frequency "推文计数") | [Go](problems/tweet-counts-per-frequency) | Medium |
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| <span id="1347">1347</span> | [Minimum Number of Steps to Make Two Strings Anagram](https://leetcode.com/problems/minimum-number-of-steps-to-make-two-strings-anagram "制造字母异位词的最小步骤数") | [Go](problems/minimum-number-of-steps-to-make-two-strings-anagram) | Medium |

‎problems/construct-target-array-with-multiple-sums/README.md

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<!--|This file generated by command(leetcode description); DO NOT EDIT. |-->
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<!--+----------------------------------------------------------------------+-->
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<!--|@author openset <[email protected]> |-->
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<!--|@link https://github.com/openset |-->
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<!--|@home https://github.com/openset/leetcode |-->
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<!--+----------------------------------------------------------------------+-->
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[< Previous](../maximum-number-of-events-that-can-be-attended "Maximum Number of Events That Can Be Attended")
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Next >
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## [1354. Construct Target Array With Multiple Sums (Hard)](https://leetcode.com/problems/construct-target-array-with-multiple-sums "多次求和构造目标数组")
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<p>Given an array of integers&nbsp;<code>target</code>. From a starting array, <code>A</code>&nbsp;consisting of all 1&#39;s, you may perform the following procedure :</p>
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<ul>
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<li>let <code>x</code> be the sum of all elements currently in your array.</li>
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<li>choose index <code>i</code>, such that&nbsp;<code>0 &lt;= i &lt; target.size</code> and set the value of <code>A</code> at index <code>i</code> to <code>x</code>.</li>
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<li>You may repeat this procedure&nbsp;as many times as needed.</li>
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</ul>
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<p>Return True if it is possible to construct the <code>target</code> array from <code>A</code> otherwise&nbsp;return False.</p>
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<p>&nbsp;</p>
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<p><strong>Example 1:</strong></p>
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<pre>
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<strong>Input:</strong> target = [9,3,5]
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<strong>Output:</strong> true
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<strong>Explanation:</strong> Start with [1, 1, 1]
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[1, 1, 1], sum = 3 choose index 1
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[1, 3, 1], sum = 5 choose index 2
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[1, 3, 5], sum = 9 choose index 0
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[9, 3, 5] Done
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</pre>
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<p><strong>Example 2:</strong></p>
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<pre>
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<strong>Input:</strong> target = [1,1,1,2]
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<strong>Output:</strong> false
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<strong>Explanation:</strong> Impossible to create target array from [1,1,1,1].
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</pre>
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<p><strong>Example 3:</strong></p>
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<pre>
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<strong>Input:</strong> target = [8,5]
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<strong>Output:</strong> true
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</pre>
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<p>&nbsp;</p>
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<p><strong>Constraints:</strong></p>
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<ul>
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<li><code>N == target.length</code></li>
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<li><code>1 &lt;= target.length&nbsp;&lt;= 5 * 10^4</code></li>
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<li><code>1 &lt;= target[i] &lt;= 10^9</code></li>
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</ul>
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### Related Topics
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[[Greedy](../../tag/greedy/README.md)]
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### Hints
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<details>
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<summary>Hint 1</summary>
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Given that the sum is strictly increasing, the largest element in the target must be formed in the last step by adding the total sum in the previous step. Thus, we can simulate the process in a reversed way.
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</details>
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<details>
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<summary>Hint 2</summary>
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Subtract the largest with the rest of the array, and put the new element into the array. Repeat until all elements become one
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</details>

‎problems/count-negative-numbers-in-a-sorted-matrix/README.md

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<!--|This file generated by command(leetcode description); DO NOT EDIT. |-->
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<!--+----------------------------------------------------------------------+-->
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<!--|@author openset <[email protected]> |-->
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<!--|@link https://github.com/openset |-->
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<!--|@home https://github.com/openset/leetcode |-->
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<!--+----------------------------------------------------------------------+-->
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[< Previous](../students-with-invalid-departments "Students With Invalid Departments")
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[Next >](../product-of-the-last-k-numbers "Product of the Last K Numbers")
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## [1351. Count Negative Numbers in a Sorted Matrix (Easy)](https://leetcode.com/problems/count-negative-numbers-in-a-sorted-matrix "统计有序矩阵中的负数")
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<p>Given a <code>m&nbsp;* n</code>&nbsp;matrix <code>grid</code>&nbsp;which is sorted in non-increasing order both row-wise and column-wise.&nbsp;</p>
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<p>Return the number of <strong>negative</strong> numbers in&nbsp;<code>grid</code>.</p>
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<p>&nbsp;</p>
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<p><strong>Example 1:</strong></p>
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<pre>
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<strong>Input:</strong> grid = [[4,3,2,-1],[3,2,1,-1],[1,1,-1,-2],[-1,-1,-2,-3]]
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<strong>Output:</strong> 8
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<strong>Explanation:</strong> There are 8 negatives number in the matrix.
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</pre>
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<p><strong>Example 2:</strong></p>
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<pre>
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<strong>Input:</strong> grid = [[3,2],[1,0]]
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<strong>Output:</strong> 0
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</pre>
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<p><strong>Example 3:</strong></p>
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<pre>
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<strong>Input:</strong> grid = [[1,-1],[-1,-1]]
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<strong>Output:</strong> 3
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</pre>
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<p><strong>Example 4:</strong></p>
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<pre>
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<strong>Input:</strong> grid = [[-1]]
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<strong>Output:</strong> 1
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</pre>
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<p>&nbsp;</p>
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<p><strong>Constraints:</strong></p>
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<ul>
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<li><code>m == grid.length</code></li>
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<li><code>n == grid[i].length</code></li>
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<li><code>1 &lt;= m, n &lt;= 100</code></li>
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<li><code>-100 &lt;= grid[i][j] &lt;= 100</code></li>
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</ul>
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### Related Topics
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[[Array](../../tag/array/README.md)]
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[[Binary Search](../../tag/binary-search/README.md)]
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### Hints
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<details>
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<summary>Hint 1</summary>
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Use binary search for optimization or simply brute force.
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</details>

‎problems/counting-bits/README.md

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</ul>
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### Related Topics
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[[Dynamic Programming](../../tag/dynamic-programming/README.md)]
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[[Bit Manipulation](../../tag/bit-manipulation/README.md)]
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[[Dynamic Programming](../../tag/dynamic-programming/README.md)]
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### Similar Questions
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1. [Number of 1 Bits](../number-of-1-bits) (Easy)

‎problems/intersection-of-two-arrays/README.md

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<p>&nbsp;</p>
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### Related Topics
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[[Sort](../../tag/sort/README.md)]
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[[Hash Table](../../tag/hash-table/README.md)]
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[[Two Pointers](../../tag/two-pointers/README.md)]
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[[Binary Search](../../tag/binary-search/README.md)]
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[[Sort](../../tag/sort/README.md)]
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### Similar Questions
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1. [Intersection of Two Arrays II](../intersection-of-two-arrays-ii) (Easy)

‎problems/maximum-number-of-events-that-can-be-attended/README.md

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<!--|This file generated by command(leetcode description); DO NOT EDIT. |-->
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<!--+----------------------------------------------------------------------+-->
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<!--|@author openset <[email protected]> |-->
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<!--|@link https://github.com/openset |-->
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<!--|@home https://github.com/openset/leetcode |-->
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<!--+----------------------------------------------------------------------+-->
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[< Previous](../product-of-the-last-k-numbers "Product of the Last K Numbers")
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[Next >](../construct-target-array-with-multiple-sums "Construct Target Array With Multiple Sums")
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## [1353. Maximum Number of Events That Can Be Attended (Medium)](https://leetcode.com/problems/maximum-number-of-events-that-can-be-attended "最多可以参加的会议数目")
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<p>Given an array of <code>events</code> where <code>events[i] = [startDay<sub>i</sub>, endDay<sub>i</sub>]</code>. Every event <code>i</code> starts at&nbsp;<code>startDay<sub>i</sub></code><sub>&nbsp;</sub>and ends at&nbsp;<code>endDay<sub>i</sub></code>.</p>
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<p>You can attend an event <code>i</code>&nbsp;at any day&nbsp;<code>d</code> where&nbsp;<code>startTime<sub>i</sub>&nbsp;&lt;= d &lt;= endTime<sub>i</sub></code>. Notice that you can only attend one event at any time <code>d</code>.</p>
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<p>Return <em>the maximum number of events&nbsp;</em>you can attend.</p>
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<p>&nbsp;</p>
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<p><strong>Example 1:</strong></p>
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<img alt="" src="https://assets.leetcode.com/uploads/2020/02/05/e1.png" style="width: 660px; height: 440px;" />
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<pre>
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<strong>Input:</strong> events = [[1,2],[2,3],[3,4]]
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<strong>Output:</strong> 3
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<strong>Explanation:</strong> You can attend all the three events.
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One way to attend them all is as shown.
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Attend the first event on day 1.
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Attend the second event on day 2.
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Attend the third event on day 3.
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</pre>
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<p><strong>Example 2:</strong></p>
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<pre>
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<strong>Input:</strong> events= [[1,2],[2,3],[3,4],[1,2]]
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<strong>Output:</strong> 4
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</pre>
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<p><strong>Example 3:</strong></p>
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<pre>
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<strong>Input:</strong> events = [[1,4],[4,4],[2,2],[3,4],[1,1]]
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<strong>Output:</strong> 4
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</pre>
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<p><strong>Example 4:</strong></p>
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<pre>
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<strong>Input:</strong> events = [[1,100000]]
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<strong>Output:</strong> 1
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</pre>
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<p><strong>Example 5:</strong></p>
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<pre>
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<strong>Input:</strong> events = [[1,1],[1,2],[1,3],[1,4],[1,5],[1,6],[1,7]]
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<strong>Output:</strong> 7
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</pre>
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<p>&nbsp;</p>
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<p><strong>Constraints:</strong></p>
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<ul>
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<li><code>1 &lt;= events.length &lt;= 10^5</code></li>
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<li><code>events[i].length == 2</code></li>
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<li><code>1 &lt;= events[i][0] &lt;= events[i][1] &lt;= 10^5</code></li>
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</ul>
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### Related Topics
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[[Greedy](../../tag/greedy/README.md)]
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[[Sort](../../tag/sort/README.md)]
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[[Segment Tree](../../tag/segment-tree/README.md)]
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### Hints
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<details>
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<summary>Hint 1</summary>
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Sort the events by the start time and in case of tie by the end time in ascending order.
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</details>
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<details>
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<summary>Hint 2</summary>
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Loop over the sorted events. Attend as much as you can and keep the last day occupied. When you try to attend new event keep in mind the first day you can attend a new event in.
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</details>

‎problems/maximum-students-taking-exam/README.md

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[< Previous](../tweet-counts-per-frequency "Tweet Counts Per Frequency")
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Next >
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[Next >](../students-with-invalid-departments "Students With Invalid Departments")
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## [1349. Maximum Students Taking Exam (Hard)](https://leetcode.com/problems/maximum-students-taking-exam "参加考试的最大学生数")
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‎problems/product-of-the-last-k-numbers/README.md

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<!--|This file generated by command(leetcode description); DO NOT EDIT. |-->
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<!--+----------------------------------------------------------------------+-->
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<!--|@author openset <[email protected]> |-->
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<!--|@link https://github.com/openset |-->
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<!--|@home https://github.com/openset/leetcode |-->
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<!--+----------------------------------------------------------------------+-->
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[< Previous](../count-negative-numbers-in-a-sorted-matrix "Count Negative Numbers in a Sorted Matrix")
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[Next >](../maximum-number-of-events-that-can-be-attended "Maximum Number of Events That Can Be Attended")
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## [1352. Product of the Last K Numbers (Medium)](https://leetcode.com/problems/product-of-the-last-k-numbers "最后 K 个数的乘积")
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<p>Implement the class <code>ProductOfNumbers</code>&nbsp;that supports two methods:</p>
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<p>1.<code>&nbsp;add(int num)</code></p>
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<ul>
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<li>Adds the number <code>num</code> to the back of the current list of numbers.</li>
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</ul>
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<p>2.<code> getProduct(int k)</code></p>
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<ul>
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<li>Returns the product of the last <code>k</code> numbers in the current list.</li>
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<li>You can assume that always the current list has <strong>at least</strong> <code>k</code> numbers.</li>
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</ul>
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<p>At any time, the product of any contiguous sequence of numbers will fit into a single 32-bit integer without overflowing.</p>
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<p>&nbsp;</p>
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<p><strong>Example:</strong></p>
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<pre>
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<strong>Input</strong>
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[&quot;ProductOfNumbers&quot;,&quot;add&quot;,&quot;add&quot;,&quot;add&quot;,&quot;add&quot;,&quot;add&quot;,&quot;getProduct&quot;,&quot;getProduct&quot;,&quot;getProduct&quot;,&quot;add&quot;,&quot;getProduct&quot;]
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[[],[3],[0],[2],[5],[4],[2],[3],[4],[8],[2]]
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<strong>Output</strong>
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[null,null,null,null,null,null,20,40,0,null,32]
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<strong>Explanation</strong>
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ProductOfNumbers productOfNumbers = new ProductOfNumbers();
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productOfNumbers.add(3); // [3]
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productOfNumbers.add(0); // [3,0]
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productOfNumbers.add(2); // [3,0,2]
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productOfNumbers.add(5); // [3,0,2,5]
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productOfNumbers.add(4); // [3,0,2,5,4]
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productOfNumbers.getProduct(2); // return 20. The product of the last 2 numbers is 5 * 4 = 20
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productOfNumbers.getProduct(3); // return 40. The product of the last 3 numbers is 2 * 5 * 4 = 40
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productOfNumbers.getProduct(4); // return 0. The product of the last 4 numbers is 0 * 2 * 5 * 4 = 0
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productOfNumbers.add(8); // [3,0,2,5,4,8]
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productOfNumbers.getProduct(2); // return 32. The product of the last 2 numbers is 4 * 8 = 32
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</pre>
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<p>&nbsp;</p>
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<p><strong>Constraints:</strong></p>
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<ul>
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<li>There will be at most <code>40000</code>&nbsp;operations considering both <code>add</code> and <code>getProduct</code>.</li>
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<li><code>0 &lt;= num&nbsp;&lt;=&nbsp;100</code></li>
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<li><code>1 &lt;= k &lt;= 40000</code></li>
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</ul>
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### Related Topics
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[[Design](../../tag/design/README.md)]
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[[Array](../../tag/array/README.md)]
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### Hints
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<details>
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<summary>Hint 1</summary>
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Keep all prefix products of numbers in an array, then calculate the product of last K elements in O(1) complexity.
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</details>
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<details>
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<summary>Hint 2</summary>
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When a zero number is added, clean the array of prefix products.
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</details>

‎problems/students-with-invalid-departments/README.md

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<!--|This file generated by command(leetcode description); DO NOT EDIT. |-->
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<!--+----------------------------------------------------------------------+-->
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<!--|@author openset <[email protected]> |-->
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<!--|@link https://github.com/openset |-->
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<!--|@home https://github.com/openset/leetcode |-->
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<!--+----------------------------------------------------------------------+-->
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[< Previous](../maximum-students-taking-exam "Maximum Students Taking Exam")
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[Next >](../count-negative-numbers-in-a-sorted-matrix "Count Negative Numbers in a Sorted Matrix")
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## [1350. Students With Invalid Departments (Easy)](https://leetcode.com/problems/students-with-invalid-departments "")
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‎problems/students-with-invalid-departments/mysql_schemas.sql

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Create table If Not Exists Departments (id int, name varchar(30));
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Create table If Not Exists Students (id int, name varchar(30), department_id int);
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Truncate table Departments;
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insert into Departments (id, name) values ('1', 'Electrical Engineering');
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insert into Departments (id, name) values ('7', 'Computer Engineering');
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insert into Departments (id, name) values ('13', 'Bussiness Administration');
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Truncate table Students;
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insert into Students (id, name, department_id) values ('23', 'Alice', '1');
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insert into Students (id, name, department_id) values ('1', 'Bob', '7');
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insert into Students (id, name, department_id) values ('5', 'Jennifer', '13');
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insert into Students (id, name, department_id) values ('2', 'John', '14');
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insert into Students (id, name, department_id) values ('4', 'Jasmine', '77');
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insert into Students (id, name, department_id) values ('3', 'Steve', '74');
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insert into Students (id, name, department_id) values ('6', 'Luis', '1');
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insert into Students (id, name, department_id) values ('8', 'Jonathan', '7');
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insert into Students (id, name, department_id) values ('7', 'Daiana', '33');
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insert into Students (id, name, department_id) values ('11', 'Madelynn', '1');

‎tag/array/README.md

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| # | 题目 | 标签 | 难度 |
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| :-: | - | - | :-: |
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| 1352 | [最后 K 个数的乘积](../../problems/product-of-the-last-k-numbers) | [[设计](../design/README.md)] [[数组](../array/README.md)] | Medium |
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| 1351 | [统计有序矩阵中的负数](../../problems/count-negative-numbers-in-a-sorted-matrix) | [[数组](../array/README.md)] [[二分查找](../binary-search/README.md)] | Easy |
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| 1346 | [检查整数及其两倍数是否存在](../../problems/check-if-n-and-its-double-exist) | [[数组](../array/README.md)] | Easy |
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| 1343 | [大小为 K 且平均值大于等于阈值的子数组数目](../../problems/number-of-sub-arrays-of-size-k-and-average-greater-than-or-equal-to-threshold) | [[数组](../array/README.md)] | Medium |
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| 1338 | [数组大小减半](../../problems/reduce-array-size-to-the-half) | [[贪心算法](../greedy/README.md)] [[数组](../array/README.md)] | Medium |

‎tag/binary-search/README.md

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| # | 题目 | 标签 | 难度 |
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| :-: | - | - | :-: |
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| 1351 | [统计有序矩阵中的负数](../../problems/count-negative-numbers-in-a-sorted-matrix) | [[数组](../array/README.md)] [[二分查找](../binary-search/README.md)] | Easy |
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| 1337 | [方阵中战斗力最弱的 K 行](../../problems/the-k-weakest-rows-in-a-matrix) | [[数组](../array/README.md)] [[二分查找](../binary-search/README.md)] | Easy |
1314
| 1300 | [转变数组后最接近目标值的数组和](../../problems/sum-of-mutated-array-closest-to-target) | [[数组](../array/README.md)] [[二分查找](../binary-search/README.md)] | Medium |
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| 1292 | [元素和小于等于阈值的正方形的最大边长](../../problems/maximum-side-length-of-a-square-with-sum-less-than-or-equal-to-threshold) | [[数组](../array/README.md)] [[二分查找](../binary-search/README.md)] | Medium |

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