LeetCode 107. Binary Tree Level Order Traversal II

/** * 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 {private:    vector<vector<int>> vec;    int max_level= 0;    void dfs(TreeNode* root,int level)    {        if(root == NULL) return;        if(vec.size() < level)        {            max_level = level;            vector<int> v;            vec.insert(vec.begin(),v);        }        vec[max_level - level].push_back(root->val);        dfs(root->left,level+1);        dfs(root->right,level+1);    }public:    vector<vector<int>> levelOrderBottom(TreeNode* root) {                dfs(root,1);        return vec;    }};