LeetCode-M-Binary Tree Level Order Traversal
2017-04-24 09:51:15         来源：hfsmurf的博客

## 题意

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],

```    3
/ \
9  20
/  \
15   7```

return its level order traversal as:

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

## 实现

```/**
* Definition for a binary tree node.
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
vector> levelOrder(TreeNode* root) {
vector> result;
if(root == NULL) return result;
queue tq;
tq.push(root);
tq.push(NULL);
while(!tq.empty() && tq.front() != NULL){
vector sr;
while(tq.front() != NULL){
TreeNode* node = tq.front();
tq.pop();
sr.push_back(node->val);
if(node->left != NULL) tq.push(node->left);
if(node->right != NULL) tq.push(node->right);
}
tq.push(NULL);
result.push_back(sr);
tq.pop();
}
return result;
}
};```