leetcode 508. Most Frequent Subtree Sum

古城微笑少年丶 2022-06-13 02:26 121阅读 0赞

# 1.题目 #

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.  
给一棵二叉树，要求返回出现次数最多的子树和。如果有多个和出现次数相同，则都返回，不计较顺序。  
假设所有和均在INT32范围内  
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.

# 2.分析 #

后序遍历，计算子树和。  
unordered\_map 记录子树和出现的次数

# 3.代码 #

class Solution {
    public:
        int postOrder(TreeNode* root, unordered_map<int, int>& counts,int& maxC) {
            int sum = root->val;
            if (root->left)
                sum += postOrder(root->left, counts, maxC);
            if (root->right)
                sum += postOrder(root->right, counts, maxC);
            counts[sum]++;
            maxC = maxC < counts[sum] ? counts[sum] : maxC;
            return sum;
        }
        vector<int> findFrequentTreeSum(TreeNode* root) {
            vector<int> ans;
            if (root == NULL)
                return ans;
            unordered_map<int, int> counts;
            int maxC = INT_MIN;
            postOrder(root, counts,maxC);
            for (auto p : counts) {
                if (p.second == maxC)
                    ans.push_back(p.first);
            }
            return ans;
        }
    };