102. Binary Tree Level Order Traversal 解题记录

题目描述：

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]
]

解题思路：

这题可以利用递归一层一层往下搜索，添加每一层的数据。

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:
    void select(vector<vector<int>> &ret, TreeNode* root, int level){
        if(!root)
        //节点不存在则返回
            return;
        if(ret.size() == level)
        //加入新的数组
            ret.push_back(vector<int>(0));
        ret[level].push_back(root->val);
        if(root->left||root->right){
            select(ret, root->left, level+1);//向左遍历 
            select(ret, root->right, level+1);//向右遍历
        }
    }
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> ret;
        select(ret, root, 0);
        return ret;
    }
};