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]

,

3
/ \
9  20
/  \
15   7

return its level order traversal as:

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

二叉树按层遍历输出，记住使用栈或队列数据结构。
/**
* 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<vector<int>> levelOrder(TreeNode* root) {
vector<vector<int> > res;
queue<TreeNode*> queue;
if(!root) return res;
queue.push(root);

while(!queue.empty())
{
vector<int>level;
int size = queue.size();
while(size--)
{
TreeNode* temp=queue.front();
queue.pop();
level.push_back(temp->val);
if(temp->left) queue.push(temp->left);
if(temp->right) queue.push(temp->right);
}
res.push_back(level);
}
return res;
}
};