✨feat: add 508

Author: SharingSource

Index/DFS.md

@@ -19,6 +19,7 @@
 | [473. 火柴拼正方形](https://leetcode.cn/problems/matchsticks-to-square/) | [LeetCode 题解链接](https://leetcode.cn/problems/matchsticks-to-square/solution/by-ac_oier-k8i7/) | 中等 | 🤩🤩🤩🤩     |
 | [488. 祖玛游戏](https://leetcode-cn.com/problems/zuma-game/) | [LeetCode 题解链接](https://leetcode-cn.com/problems/zuma-game/solution/gong-shui-san-xie-yi-ti-shuang-jie-sou-s-3ftb/) | 困难 | 🤩🤩🤩🤩     |
 | [494. 目标和](https://leetcode-cn.com/problems/target-sum/)  | [LeetCode 题解链接](https://leetcode-cn.com/problems/target-sum/solution/gong-shui-san-xie-yi-ti-si-jie-dfs-ji-yi-et5b/) | 中等 | 🤩🤩🤩🤩     |
+| [508. 出现次数最多的子树元素和](https://leetcode.cn/problems/most-frequent-subtree-sum/) | [LeetCode 题解链接](https://leetcode.cn/problems/most-frequent-subtree-sum/solution/by-ac_oier-t3y4/) | 中等 | 🤩🤩🤩      |
 | [559. N 叉树的最大深度](https://leetcode-cn.com/problems/maximum-depth-of-n-ary-tree/) | [LeetCode 题解链接](https://leetcode-cn.com/problems/maximum-depth-of-n-ary-tree/solution/gong-shui-san-xie-yi-ti-shuang-jie-dfs-b-n956/) | 简单 | 🤩🤩🤩🤩     |
 | [563. 二叉树的坡度](https://leetcode-cn.com/problems/binary-tree-tilt/) | [LeetCode 题解链接](https://leetcode-cn.com/problems/binary-tree-tilt/solution/gong-shui-san-xie-jian-dan-er-cha-shu-di-ekz4/) | 简单 | 🤩🤩🤩🤩     |
 | [589. N 叉树的前序遍历](https://leetcode-cn.com/problems/n-ary-tree-preorder-traversal/) | [LeetCode 题解链接](https://leetcode-cn.com/problems/n-ary-tree-preorder-traversal/solution/gong-shui-san-xie-shu-de-sou-suo-yun-yon-pse1/) | 简单 | 🤩🤩🤩      |

Index/哈希表.md

@@ -25,6 +25,7 @@
 | [460. LFU 缓存](https://leetcode-cn.com/problems/lfu-cache/) | [LeetCode 题解链接](https://leetcode-cn.com/problems/lfu-cache/solution/gong-shui-san-xie-yun-yong-tong-pai-xu-s-53m3/) | 困难 | 🤩🤩🤩🤩🤩 |
 | [496. 下一个更大元素 I](https://leetcode-cn.com/problems/next-greater-element-i/) | [LeetCode 题解链接](https://leetcode-cn.com/problems/next-greater-element-i/solution/gong-shui-san-xie-yi-ti-shuang-jie-bian-n6nwz/) | 中等 | 🤩🤩🤩🤩 |
 | [500. 键盘行](https://leetcode-cn.com/problems/keyboard-row/) | [LeetCode 题解链接](https://leetcode-cn.com/problems/keyboard-row/solution/gong-shui-san-xie-jian-dan-zi-fu-chuan-m-zx6b/) | 简单 | 🤩🤩🤩🤩 |
+| [508. 出现次数最多的子树元素和](https://leetcode.cn/problems/most-frequent-subtree-sum/) | [LeetCode 题解链接](https://leetcode.cn/problems/most-frequent-subtree-sum/solution/by-ac_oier-t3y4/) | 中等 | 🤩🤩🤩 |
 | [519. 随机翻转矩阵](https://leetcode-cn.com/problems/random-flip-matrix/) | [LeetCode 题解链接](https://leetcode-cn.com/problems/random-flip-matrix/solution/gong-shui-san-xie-note-bie-pian-yi-ti-sh-e6gi/) | 中等 | 🤩🤩🤩🤩 |
 | [532. 数组中的 k-diff 数对](https://leetcode.cn/problems/k-diff-pairs-in-an-array/) | [LeetCode 题解链接](https://leetcode.cn/problems/k-diff-pairs-in-an-array/solution/by-ac_oier-ap3v/) | 中等 | 🤩🤩🤩🤩 |
 | [554. 砖墙](https://leetcode-cn.com/problems/brick-wall/) | [LeetCode 题解链接](https://leetcode-cn.com/problems/brick-wall/solution/gong-shui-san-xie-zheng-nan-ze-fan-shi-y-gsri/) | 中等 | 🤩🤩🤩 |

LeetCode/501-510/508. 出现次数最多的子树元素和（中等）.md

@@ -0,0 +1,79 @@
+### 题目描述
+
+这是 LeetCode 上的 **[508. 出现次数最多的子树元素和](https://leetcode.cn/problems/most-frequent-subtree-sum/solution/by-ac_oier-t3y4/)** ，难度为 **中等**。
+
+Tag : 「树的遍历」、「DFS」、「哈希表」
+
+
+
+给你一个二叉树的根结点 `root`，请返回出现次数最多的子树元素和。如果有多个元素出现的次数相同，返回所有出现次数最多的子树元素和（不限顺序）。
+
+一个结点的 「子树元素和」 定义为以该结点为根的二叉树上所有结点的元素之和（包括结点本身）。
+
+示例 1：
+![](https://assets.leetcode.com/uploads/2021/04/24/freq1-tree.jpg)
+```
+输入: root = [5,2,-3]
+
+输出: [2,-3,4]
+```
+示例 2：
+![](https://assets.leetcode.com/uploads/2021/04/24/freq2-tree.jpg)
+```
+输入: root = [5,2,-5]
+
+输出: [2]
+```
+
+提示:
+* 节点数在 $[1, 10^4]$ 范围内
+* $-10^5 <= Node.val <= 10^5$
+
+---
+
+### DFS
+
+使用 `DFS` 对树进行遍历，同时将每棵子树的总和值存入哈希表（以元素和为 `key`，出现次数为 `value`），并实时维护最大子树和，使用变量 `max` 存储。
+
+当遍历完整棵树后，将哈希表中值为 `max` 的键存入答案。
+
+代码：
+```Java
+class Solution {
+    Map<Integer, Integer> map = new HashMap<>();
+    int max = 0;
+    public int[] findFrequentTreeSum(TreeNode root) {
+        dfs(root);
+        List<Integer> list = new ArrayList<>();
+        for (int k : map.keySet()) {
+            if (map.get(k) == max) list.add(k);
+        }
+        int n = list.size();
+        int[] ans = new int[n];
+        for (int i = 0; i < n; i++) ans[i] = list.get(i);
+        return ans;
+    }
+    int dfs(TreeNode root) {
+        if (root == null) return 0;
+        int cur = root.val + dfs(root.left) + dfs(root.right);
+        map.put(cur, map.getOrDefault(cur, 0) + 1);
+        max = Math.max(max, map.get(cur));
+        return cur;
+    }
+}
+```
+* 时间复杂度：$O(n)$
+* 空间复杂度：$O(n)$
+
+---
+
+### 最后
+
+这是我们「刷穿 LeetCode」系列文章的第 `No.508` 篇，系列开始于 2021/01/01，截止于起始日 LeetCode 上共有 1916 道题目，部分是有锁题，我们将先把所有不带锁的题目刷完。
+
+在这个系列文章里面，除了讲解解题思路以外，还会尽可能给出最为简洁的代码。如果涉及通解还会相应的代码模板。
+
+为了方便各位同学能够电脑上进行调试和提交代码，我建立了相关的仓库：https://github.com/SharingSource/LogicStack-LeetCode 。
+
+在仓库地址里，你可以看到系列文章的题解链接、系列文章的相应代码、LeetCode 原题链接和其他优选题解。
+