Leetcode NO.102 Binary Tree Level Order Traversal

本题题目要求如下：

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,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

return its level order traversal as:

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

思路还是很简单的，就是用dfs遍历，只不过要加上一个level参数来记录该node的层数，然后再正常的插入即可，代码如下：

/**
 * 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) {
        dfs(root, 0);
        return res;
    }
private:
    vector> res;
    void dfs(TreeNode* root, int level) {
        if (root == nullptr) {
            return;
        }
        if (res.size() == level) {
            vector arr;
            res.push_back(arr);
        }
        res[level].push_back(root->val);
        dfs(root->left, level + 1);
        dfs(root->right, level + 1);
    }
};