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Commit 20c74fe

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Merge pull request SharingSource#566 from SharingSource/ac_oier
✨feat: add 508
2 parents d5aff5d + 2ed9172 commit 20c74fe

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‎Index/DFS.md‎

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| [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 | 🤩🤩🤩🤩 |
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| [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 | 🤩🤩🤩🤩 |
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| [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 | 🤩🤩🤩🤩 |
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| [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 | 🤩🤩🤩 |
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| [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 | 🤩🤩🤩🤩 |
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| [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 | 🤩🤩🤩🤩 |
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| [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

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| [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 | 🤩🤩🤩🤩🤩 |
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| [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 | 🤩🤩🤩🤩 |
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| [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 | 🤩🤩🤩🤩 |
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| [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 | 🤩🤩🤩 |
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| [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 | 🤩🤩🤩🤩 |
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| [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 | 🤩🤩🤩🤩 |
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| [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 | 🤩🤩🤩 |
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### 题目描述
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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**。
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Tag : 「树的遍历」、「DFS」、「哈希表」
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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)。
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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)。
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示例 1:
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![](https://assets.leetcode.com/uploads/2021/04/24/freq1-tree.jpg)
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```
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输入: root = [5,2,-3]
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输出: [2,-3,4]
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```
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示例 2:
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![](https://assets.leetcode.com/uploads/2021/04/24/freq2-tree.jpg)
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```
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输入: root = [5,2,-5]
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输出: [2]
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```
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提示:
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* 节点数在 $[1, 10^4]$ 范围内
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* $-10^5 <= Node.val <= 10^5$
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---
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### DFS
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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。
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当遍历完整棵树后,将哈希表中值为 `max` 的键存入答案。
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代码:
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```Java
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class Solution {
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Map<Integer, Integer> map = new HashMap<>();
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int max = 0;
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public int[] findFrequentTreeSum(TreeNode root) {
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dfs(root);
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List<Integer> list = new ArrayList<>();
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for (int k : map.keySet()) {
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if (map.get(k) == max) list.add(k);
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}
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int n = list.size();
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int[] ans = new int[n];
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for (int i = 0; i < n; i++) ans[i] = list.get(i);
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return ans;
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}
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int dfs(TreeNode root) {
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if (root == null) return 0;
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int cur = root.val + dfs(root.left) + dfs(root.right);
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map.put(cur, map.getOrDefault(cur, 0) + 1);
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max = Math.max(max, map.get(cur));
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return cur;
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}
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}
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```
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* 时间复杂度:$O(n)$
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* 空间复杂度:$O(n)$
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---
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### 最后
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这是我们「刷穿 LeetCode」系列文章的第 `No.508` 篇,系列开始于 2021年01月01日,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。
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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。
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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 。
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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。
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