[LeetCode] 501. Find Mode in Binary Tree

https://leetcode.com/contest/leetcode-weekly-contest-17/problems/find-mode-in-binary-tree/
Find all the elements that appears mostly.

Solution:
Use a hashmap to count the time of duplicates. Then save the elements with the highest appearing time into the output.

Code:

/**
 * 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> findMode(TreeNode* root) {
        vector<int> res;
        unordered_map<int, int> m;
        int ct = 0;
        findM(root, m, ct);
        for(auto a:m){
            if(a.second == ct) res.push_back(a.first);
        }
        return res;
    }
private:
    void findM(TreeNode* root, unordered_map<int, int> &m, int &c){
        if(!root) return;
        c = max(c, ++m[root->val]);
        findM(root->left, m, c);
        findM(root->right, m, c);
    }
};