leetcode 107.Binary Tree Level Order Traversal II 二叉树的层次遍历 II

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     vector<vector<int>> levelOrderBottom(TreeNode* root) {
13         if(root==NULL) return {};
14         queue<TreeNode*> q;
15         TreeNode* front;
16         q.push(root);
17         vector<vector<int>> res;
18         while(!q.empty()){
19             vector<int> onelevel;
20             for(int i=q.size();i>0;i--){
21                 front=q.front();
22                 q.pop();
23                 if(front->left)
24                     q.push(front->left);
25                 if(front->right)
26                     q.push(front->right);
27                 onelevel.push_back(front->val);
28             }
29             res.push_back(onelevel);
30         }
31         reverse(res.begin(),res.end());
32         return res;
33     }
34 };