102. Binary Tree Level Order Traversal

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example:
Given binary tree [3,9,20,null,null,15,7],

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]

层次遍历

C++：

 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>> levelOrder(TreeNode* root) {
13         if (root == NULL)
14             return vector<vector<int>>() ;
15         vector<vector<int>> res ;
16         vector<int> level ;
17         queue<TreeNode*> q ;
18         q.push(root) ;
19         while(!q.empty()){
20             int len = q.size() ;
21             for(int i = 0 ; i < len ; i++){
22                 TreeNode* temp = q.front() ;
23                 q.pop() ;
24                 if (temp->left != NULL)
25                     q.push(temp->left) ;
26                 if (temp->right != NULL)
27                     q.push(temp->right) ;
28                 level.push_back(temp->val) ;
29             }
30             res.push_back(level) ;
31             level.clear() ;
32         }
33         return res;
34     }
35 };