Merge pull request #577 from openset/develop

Add: Related Topics

Author: Shuo

problems/adding-two-negabinary-numbers/README.md

@@ -38,3 +38,12 @@
 	<li><code>arr1[i]</code> is <code>0</code> or <code>1</code></li>
 	<li><code>arr2[i]</code> is <code>0</code> or <code>1</code></li>
 </ol>
+
+### Related Topics
+  [[Math](https://github.com/openset/leetcode/tree/master/tag/math/README.md)]
+
+### Hints
+<details>
+<summary>Hint 1</summary>
+We can try to determine the last digit of the answer, then divide everything by 2 and repeat.
+</details>

problems/flip-columns-for-maximum-number-of-equal-rows/README.md

@@ -60,3 +60,12 @@
 </div>
 </div>
 </div>
+
+### Related Topics
+  [[Hash Table](https://github.com/openset/leetcode/tree/master/tag/hash-table/README.md)]
+
+### Hints
+<details>
+<summary>Hint 1</summary>
+Flipping a subset of columns is like doing a bitwise XOR of some number K onto each row.  We want rows X with X ^ K = all 0s or all 1s.  This is the same as X = X^K ^K = (all 0s or all 1s) ^ K, so we want to count rows that have opposite bits set.  For example, if K = 1, then we count rows X = (00000...001, or 1111....110).
+</details>

problems/greatest-common-divisor-of-strings/README.md

@@ -9,7 +9,7 @@
 
 [Next >](https://github.com/openset/leetcode/tree/master/problems/flip-columns-for-maximum-number-of-equal-rows "Flip Columns For Maximum Number of Equal Rows")
 
-## 5076. Greatest Common Divisor of Strings (Easy)
+## 1071. Greatest Common Divisor of Strings (Easy)
 
 <p>For strings <code>S</code> and <code>T</code>, we say &quot;<code>T</code> divides <code>S</code>&quot; if and only if <code>S = T + ... + T</code>&nbsp; (<code>T</code> concatenated with itself 1 or more times)</p>
 
@@ -47,3 +47,12 @@
 	<li><code>1 &lt;= str2.length &lt;= 1000</code></li>
 	<li><code>str1[i]</code> and <code>str2[i]</code> are English uppercase letters.</li>
 </ol>
+
+### Related Topics
+  [[String](https://github.com/openset/leetcode/tree/master/tag/string/README.md)]
+
+### Hints
+<details>
+<summary>Hint 1</summary>
+The greatest common divisor must be a prefix of each string, so we can try all prefixes.
+</details>

problems/number-of-submatrices-that-sum-to-target/README.md

@@ -47,3 +47,15 @@ Next >
 	<li><code>-1000 &lt;= matrix[i] &lt;= 1000</code></li>
 	<li><code>-10^8 &lt;= target &lt;= 10^8</code></li>
 </ol>
+
+### Related Topics
+  [[Array](https://github.com/openset/leetcode/tree/master/tag/array/README.md)]
+  [[Dynamic Programming](https://github.com/openset/leetcode/tree/master/tag/dynamic-programming/README.md)]
+  [[Sliding Window](https://github.com/openset/leetcode/tree/master/tag/sliding-window/README.md)]
+
+### Hints
+<details>
+<summary>Hint 1</summary>
+Using a 2D prefix sum, we can query the sum of any submatrix in O(1) time.
+Now for each (r1, r2), we can find the largest sum of a submatrix that uses every row in [r1, r2] in linear time using a sliding window.
+</details>