给定一个二叉树，返回其按层次遍历的节点值。 （即zhu‘ceng‘de，从左到右访问）。
例如:
给定二叉树: [3,9,20,null,null,15,7],
3
/ \
9 20
/ \
15 7
返回其层次遍历结果为：
[
[3],
[9,20],
[15,7]
]
详见：https://leetcode.com/problems/binary-tree-level-order-traversal/description/
/**
* 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;
if(!root)
{
return res;
}
queue<TreeNode*> que;
que.push(root);
vector<int> out;
int i=1;
while(!que.empty())
{
root=que.front();
que.pop();
--i;
out.push_back(root->val);
if(root->left)
{
que.push(root->left);
}
if(root->right)
{
que.push(root->right);
}
if(i==0)
{
i=que.size();
res.push_back(out);
out.clear();
}
}
return res;
}
};
