# 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.first);
for (int i = 1; i < vec.size(); i++) {
// 取最高的放到result数组中
if (vec[i].second == vec.second) result.push_back(vec[i].first);
else break;
}
return result;
}
};
``````