Leetcode之Binary Tree Level Order Traversal

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,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its level order traversal as:

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

代码：

/**
 * 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* > q;
	q.push(root);
	while (!q.empty()) {
		vector<TreeNode*> top;
		vector<int> v;
		while (!q.empty()) {
			v.push_back(q.front()->val);
			top.push_back(q.front());
			q.pop();
		}
		res.push_back(v);
		for (int i = 0; i < top.size(); i++) {
			if (top[i]->left) {
				q.push(top[i]->left);
			}
			if (top[i]->right) {
				q.push(top[i]->right);
			}
		}
	}
	return res;
    }
};