508.cpp

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> findFrequentTreeSum(TreeNode* root) {
        unordered_map<int, int> freqs;
        int max_freq = -1;
        vector<int> ans;
        (void)treeSum(root, freqs, max_freq, ans);
        return ans;
    }
    int treeSum(TreeNode* root, unordered_map<int, int>& freqs, int& max_freq, vector<int>& ans) {
        if (!root) return 0;
        int sum = root->val + 
            treeSum(root->left, freqs, max_freq, ans) +
            treeSum(root->right, freqs, max_freq, ans);
        int freq = ++freqs[sum];
        if (freq > max_freq) {
            max_freq = freq;
            ans.clear();
        }
        if (freq == max_freq) {
            ans.push_back(sum);
        }
        return sum;
    }
};