Binary Tree Level Order Traversal问题及解法

问题描述：

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).
示例：

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]
]

问题分析：
层次遍历，顾名思义就是树的广度优先遍历。

过程详见代码：
/**
* 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 == NULL) return res;
bl(root, res, 0);
return res;
}

void bl(TreeNode* root, vector<vector<int>>& res, int depth)
{
if (root == NULL) return;
if (res.size() <= depth)
res.push_back(vector<int>());
res[depth].push_back(root->val);
bl(root->left, res, depth + 1);
bl(root->right, res, depth + 1);
}
};