Binary Tree Level Order Traversal 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         vector<vector<int>> res;
14 
15         stack<vector<int>> st;
16         
17         if(root==NULL)
18             return res;
19         TreeNode * temp;
20         queue<TreeNode *> q;
21         q.push(root);
22         while(!q.empty())  //层间循环
23         {
24             queue<TreeNode *> q2;
25             vector<int> level;
26             while(!q.empty())  //层内的节点循环
27             {
28                 temp=q.front();
29                 q.pop();
30                 level.push_back(temp->val);
31                 if(temp->left!=NULL)
32                     q2.push(temp->left);
33                 if(temp->right!=NULL)
34                     q2.push(temp->right);
35             }
36             q=q2;
37             st.push(level);
38         }
39         while(!st.empty())
40         {
41             res.push_back(st.top());
42             st.pop();
43         }
44         return res;
45      }
46 };