LeetCode "501. Find Mode in Binary Search Tree"

It is a matter of hashmap use. For the follow-up:

typedef unordered_map<int, unsigned> HM;
class Solution {
    HM go(TreeNode *p)
    {
        if(!p) return HM();
        
        auto rl = go(p->left);
        auto rr = go(p->right);
        
        HM r = rl;
        for(auto &kv : rr) r[kv.first] += kv.second;
        r[p->val] ++;
        
        return r;
    }
public:
    vector<int> findMode(TreeNode* root) 
    {
        auto h = go(root);

        int mcnt = 0;
        vector<int> ret;
    
        for(auto &kv : h)
        {
            if(kv.second > mcnt)
            {
                mcnt = kv.second;
                ret = {kv.first};
            }
            else if(kv.second == mcnt)
            {
                ret.push_back(kv.first);
            }
        }
    
        return ret;
    }
};