Map to vector pair map.second sort

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;
    }
};

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