A: new

Author: Shuo

README.md

@@ -78,6 +78,22 @@ LeetCode Problems' Solutions
 
 | # | Title | Solution | Difficulty |
 | :-: | - | - | :-: |
+| <span id="2004">2004</span> | [The Number of Seniors and Juniors to Join the Company](https://leetcode.com/problems/the-number-of-seniors-and-juniors-to-join-the-company) 🔒 | [MySQL](problems/the-number-of-seniors-and-juniors-to-join-the-company) | Hard |
+| <span id="2003">2003</span> | [Smallest Missing Genetic Value in Each Subtree](https://leetcode.com/problems/smallest-missing-genetic-value-in-each-subtree "每棵子树内缺失的最小基因值") | [Go](problems/smallest-missing-genetic-value-in-each-subtree) | Hard |
+| <span id="2002">2002</span> | [Maximum Product of the Length of Two Palindromic Subsequences](https://leetcode.com/problems/maximum-product-of-the-length-of-two-palindromic-subsequences "两个回文子序列长度的最大乘积") | [Go](problems/maximum-product-of-the-length-of-two-palindromic-subsequences) | Medium |
+| <span id="2001">2001</span> | [Number of Pairs of Interchangeable Rectangles](https://leetcode.com/problems/number-of-pairs-of-interchangeable-rectangles "可互换矩形的组数") | [Go](problems/number-of-pairs-of-interchangeable-rectangles) | Medium |
+| <span id="2000">2000</span> | [Reverse Prefix of Word](https://leetcode.com/problems/reverse-prefix-of-word "反转单词前缀") | [Go](problems/reverse-prefix-of-word) | Easy |
+| <span id="1999">1999</span> | [Smallest Greater Multiple Made of Two Digits](https://leetcode.com/problems/smallest-greater-multiple-made-of-two-digits) 🔒 | [Go](problems/smallest-greater-multiple-made-of-two-digits) | Medium |
+| <span id="1998">1998</span> | [GCD Sort of an Array](https://leetcode.com/problems/gcd-sort-of-an-array "数组的最大公因数排序") | [Go](problems/gcd-sort-of-an-array) | Hard |
+| <span id="1997">1997</span> | [First Day Where You Have Been in All the Rooms](https://leetcode.com/problems/first-day-where-you-have-been-in-all-the-rooms "访问完所有房间的第一天") | [Go](problems/first-day-where-you-have-been-in-all-the-rooms) | Medium |
+| <span id="1996">1996</span> | [The Number of Weak Characters in the Game](https://leetcode.com/problems/the-number-of-weak-characters-in-the-game "游戏中弱角色的数量") | [Go](problems/the-number-of-weak-characters-in-the-game) | Medium |
+| <span id="1995">1995</span> | [Count Special Quadruplets](https://leetcode.com/problems/count-special-quadruplets "统计特殊四元组") | [Go](problems/count-special-quadruplets) | Easy |
+| <span id="1994">1994</span> | [The Number of Good Subsets](https://leetcode.com/problems/the-number-of-good-subsets "好子集的数目") | [Go](problems/the-number-of-good-subsets) | Hard |
+| <span id="1993">1993</span> | [Operations on Tree](https://leetcode.com/problems/operations-on-tree "树上的操作") | [Go](problems/operations-on-tree) | Medium |
+| <span id="1992">1992</span> | [Find All Groups of Farmland](https://leetcode.com/problems/find-all-groups-of-farmland "找到所有的农场组") | [Go](problems/find-all-groups-of-farmland) | Medium |
+| <span id="1991">1991</span> | [Find the Middle Index in Array](https://leetcode.com/problems/find-the-middle-index-in-array "找到数组的中间位置") | [Go](problems/find-the-middle-index-in-array) | Easy |
+| <span id="1990">1990</span> | [Count the Number of Experiments](https://leetcode.com/problems/count-the-number-of-experiments) 🔒 | [MySQL](problems/count-the-number-of-experiments) | Medium |
+| <span id="1989">1989</span> | [Maximum Number of People That Can Be Caught in Tag](https://leetcode.com/problems/maximum-number-of-people-that-can-be-caught-in-tag) 🔒 | [Go](problems/maximum-number-of-people-that-can-be-caught-in-tag) | Medium |
 | <span id="1988">1988</span> | [Find Cutoff Score for Each School](https://leetcode.com/problems/find-cutoff-score-for-each-school) 🔒 | [MySQL](problems/find-cutoff-score-for-each-school) | Medium |
 | <span id="1987">1987</span> | [Number of Unique Good Subsequences](https://leetcode.com/problems/number-of-unique-good-subsequences "不同的好子序列数目") | [Go](problems/number-of-unique-good-subsequences) | Hard |
 | <span id="1986">1986</span> | [Minimum Number of Work Sessions to Finish the Tasks](https://leetcode.com/problems/minimum-number-of-work-sessions-to-finish-the-tasks "完成任务的最少工作时间段") | [Go](problems/minimum-number-of-work-sessions-to-finish-the-tasks) | Medium |

problems/4sum/README.md

@@ -54,3 +54,4 @@
   1. [Two Sum](../two-sum) (Easy)
   1. [3Sum](../3sum) (Medium)
   1. [4Sum II](../4sum-ii) (Medium)
+  1. [Count Special Quadruplets](../count-special-quadruplets) (Easy)

problems/all-paths-from-source-to-target/README.md

@@ -65,7 +65,10 @@
 </ul>
 
 ### Related Topics
+  [[Backtracking](../../tag/backtracking/README.md)]
   [[Depth-First Search](../../tag/depth-first-search/README.md)]
   [[Breadth-First Search](../../tag/breadth-first-search/README.md)]
   [[Graph](../../tag/graph/README.md)]
-  [[Backtracking](../../tag/backtracking/README.md)]
+
+### Similar Questions
+  1. [Number of Ways to Arrive at Destination](../number-of-ways-to-arrive-at-destination) (Medium)

problems/arithmetic-subarrays/README.md

@@ -64,6 +64,10 @@ In the 2<sup>nd</sup> query, the subarray is <code>[5,9,3,7]. This</code> can be
   [[Array](../../tag/array/README.md)]
   [[Sorting](../../tag/sorting/README.md)]
 
+### Similar Questions
+  1. [Arithmetic Slices](../arithmetic-slices) (Medium)
+  1. [Can Make Arithmetic Progression From Sequence](../can-make-arithmetic-progression-from-sequence) (Easy)
+
 ### Hints
 <details>
 <summary>Hint 1</summary>

problems/average-of-levels-in-binary-tree/README.md

@@ -16,7 +16,7 @@ Given the <code>root</code> of a binary tree, return <em>the average value of th
 <p><strong>Example 1:</strong></p>
 <img alt="" src="https://assets.leetcode.com/uploads/2021/03/09/avg1-tree.jpg" style="width: 277px; height: 302px;" />
 <pre>
-<strong>Input:</strong> root = [3,9,20,null,15,7]
+<strong>Input:</strong> root = [3,9,20,null,null,15,7]
 <strong>Output:</strong> [3.00000,14.50000,11.00000]
 Explanation: The average value of nodes on level 0 is 3, on level 1 is 14.5, and on level 2 is 11.
 Hence return [3, 14.5, 11].

problems/avoid-flood-in-the-city/README.md

@@ -93,10 +93,10 @@ After that, it will rain over lakes [1,2]. It's easy to prove that no matter
 </ul>
 
 ### Related Topics
-  [[Greedy](../../tag/greedy/README.md)]
   [[Array](../../tag/array/README.md)]
   [[Hash Table](../../tag/hash-table/README.md)]
   [[Binary Search](../../tag/binary-search/README.md)]
+  [[Greedy](../../tag/greedy/README.md)]
   [[Heap (Priority Queue)](../../tag/heap-priority-queue/README.md)]
 
 ### Hints

problems/best-position-for-a-service-centre/README.md

@@ -71,8 +71,8 @@ Be careful with the precision!
 </ul>
 
 ### Related Topics
-  [[Geometry](../../tag/geometry/README.md)]
   [[Math](../../tag/math/README.md)]
+  [[Geometry](../../tag/geometry/README.md)]
   [[Randomized](../../tag/randomized/README.md)]
 
 ### Hints

problems/binary-search-tree-iterator-ii/README.md

@@ -21,6 +21,9 @@
   [[Binary Tree](../../tag/binary-tree/README.md)]
   [[Iterator](../../tag/iterator/README.md)]
 
+### Similar Questions
+  1. [Binary Search Tree Iterator](../binary-search-tree-iterator) (Medium)
+
 ### Hints
 <details>
 <summary>Hint 1</summary>

problems/build-an-array-with-stack-operations/README.md

@@ -68,8 +68,8 @@ Read number 3 and automatically push in the array -> [1,3]
 </ul>
 
 ### Related Topics
-  [[Stack](../../tag/stack/README.md)]
   [[Array](../../tag/array/README.md)]
+  [[Stack](../../tag/stack/README.md)]
   [[Simulation](../../tag/simulation/README.md)]
 
 ### Hints

problems/bulb-switcher-iv/README.md

@@ -69,8 +69,8 @@ We need at least 3 flip operations to form target.</pre>
 </ul>
 
 ### Related Topics
-  [[Greedy](../../tag/greedy/README.md)]
   [[String](../../tag/string/README.md)]
+  [[Greedy](../../tag/greedy/README.md)]
 
 ### Hints
 <details>