LeetCode #508. Most Frequent Subtree Sum

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本文介绍了一种算法，用于找到给定树中出现频率最高的子树和。通过递归遍历树的节点，计算每个子树的和，并使用哈希表记录每个和的出现次数。最后，返回出现次数最多的子树和值。如果存在多个和值具有相同的最高频率，则全部返回。

题目描述：

Given the root of a tree, you are asked to find the most frequent subtree sum. The subtree sum of a node is defined as the sum of all the node values formed by the subtree rooted at that node (including the node itself). So what is the most frequent subtree sum value? If there is a tie, return all the values with the highest frequency in any order.

Examples 1
Input:

  5
 /  \
2   -3

return [2, -3, 4], since all the values happen only once, return all of them in any order.

Examples 2
Input:

  5
 /  \
2   -5

return [2], since 2 happens twice, however -5 only occur once.

Note: You may assume the sum of values in any subtree is in the range of 32-bit signed integer.

class Solution {
public:
    vector<int> findFrequentTreeSum(TreeNode* root) {
        unordered_map<int,int> count;
        get_subtree_sum(root,count);
        int max_count=0;
        for(auto x:count) max_count=max(max_count,x.second);
        vector<int> result;
        for(auto x:count) if(x.second==max_count) result.push_back(x.first);
        return result;
    }
    
    int get_subtree_sum(TreeNode* node, unordered_map<int,int>& count)
    {
        if(node==NULL) return 0;
        int left=get_subtree_sum(node->left,count);
        int right=get_subtree_sum(node->right,count);
        int sum=left+right+node->val;
        count[sum]++;
        return sum;
    }
};