LeetCode OJ

题目：

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,#,#,15,7},

return its level order traversal as:

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

解题思路：

广度优先遍历。

代码：

 1 /**
 2  * Definition for binary tree
 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         vector<vector<int>> ans;
14         if (root == NULL) return ans;
15 
16         queue<TreeNode *> one, another;
17         one.push(root);
18 
19         while (!one.empty() || !another.empty()) {
20             if (!one.empty()) {
21                 vector<int> cur_ans;
22                 while (!one.empty()) {
23                     TreeNode * node = one.front();
24                     cur_ans.push_back(node->val);
25                     if (node->left != NULL) another.push(node->left);
26                     if (node->right != NULL) another.push(node->right);
27                     one.pop();
28                 }
29                 ans.push_back(cur_ans);
30             }
31             if (!another.empty()) {
32                 vector<int> cur_ans;
33                 while (!another.empty()) {
34                     TreeNode *node = another.front();
35                     cur_ans.push_back(node->val);
36                     if (node->left != NULL) one.push(node->left);
37                     if (node->right != NULL) one.push(node->right);
38                     another.pop();
39                 }
40                 ans.push_back(cur_ans);
41             }
42         }
43         return ans;
44     }
45 };