23 从上往下打印二叉树

从上往下打印出二叉树的每个节点，同层节点从左至右打印。

C++:

1 /*
2 struct TreeNode {
3     int val;
4     struct TreeNode *left;
5     struct TreeNode *right;
6     TreeNode(int x) :
7             val(x), left(NULL), right(NULL) {
8     }
9 };*/
10 class Solution {
11 public:
12     vector<int> PrintFromTopToBottom(TreeNode* root) {
13         if (root == NULL)
14             return vector<int>() ;
15         queue<TreeNode*> q ;
16         vector<int> res ;
17         q.push(root) ;
18         while(!q.empty()){
19             TreeNode* node = q.front() ;
20             q.pop() ;
21             res.push_back(node->val) ;
22             if (node->left != NULL)
23                 q.push(node->left) ;
24             if (node->right != NULL)
25                 q.push(node->right) ;
26         }
27         return res ;
28     }
29 };