[leetcode]508. Most Frequent Subtree Sum

xiaocong1990

于 2017-06-10 16:19:17 发布

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本文介绍了一种算法，用于解决LeetCode上的一道题目——寻找给定树中最频繁出现的子树和。通过递归地计算每个节点的子树和，并使用哈希表来跟踪各个子树和出现的频率，最终找出出现频率最高的子树和。

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题目链接：https://leetcode.com/problems/most-frequent-subtree-sum/#/description

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.

class Solution {
public:
    vector<int> findFrequentTreeSum(TreeNode* root) {
        unordered_map<int,int> counts;
        int maxCount=0;
        countSubtreeSums(root,counts,maxCount);
        vector<int> maxSums;
        for(const auto x:counts)
        {
            if(x.second==maxCount)
                maxSums.push_back(x.first);
        }
        return maxSums;
    }

    int countSubtreeSums(TreeNode* r,unordered_map<int,int>& counts,int& maxCount)
    {
        if(r== nullptr) return 0;
        int sum=r->val;
        sum+=countSubtreeSums(r->left,counts,maxCount);
        sum+=countSubtreeSums(r->right,counts,maxCount);
        ++counts[sum];
        maxCount=max(maxCount,counts[sum]);
        return sum;
    }
};