in leetcode501, there are two little points to learn.

1. Insert element

as an array

map has several insertion methods, please refer to the link for specific differences.

void searchBST(TreeNode* cur, unordered_map<int, int>& map) { // 前序遍历
    if (cur == NULL) return ;
    map[cur->val]++; // 以数组方式插入，统计元素频率
    searchBST(cur->left, map);
    searchBST(cur->right, map);
    return ;
}

. Map into vector sort

needs to be sorted by the occurrence times, that is, by the second element of map, but map is sorted by default for first(key), so I want to convert to vector sorting. Define and initialize the statement as . pair< int, int> > vec(map.begin(), map.end()); , after custom collation sort(vec.begin(), vec.end(), CMP);

the whole code is

class Solution {
private:

void searchBST(TreeNode* cur, unordered_map<int, int>& map) { // 前序遍历
    if (cur == NULL) return ;
    map[cur->val]++; // 统计元素频率
    searchBST(cur->left, map);
    searchBST(cur->right, map);
    return ;
}
bool static cmp (const pair<int, int>& a, const pair<int, int>& b) {
    return a.second > b.second;
}
public:
    vector<int> findMode(TreeNode* root) {
        unordered_map<int, int> map; // key:元素，value:出现频率
        vector<int> result;
        if (root == NULL) return result;
        searchBST(root, map);
        vector<pair<int, int>> vec(map.begin(), map.end());
        sort(vec.begin(), vec.end(), cmp); // 给频率排个序
        result.push_back(vec[0].first);
        for (int i = 1; i < vec.size(); i++) { 
            // 取最高的放到result数组中
            if (vec[i].second == vec[0].second) result.push_back(vec[i].first);
            else break;
        }
        return result;
    }
};