[LeetCode]题解（python）：102 Binary Tree Level Order Traversal

题目来源

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https://leetcode.com/problems/binary-tree-level-order-traversal/

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

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题意分析

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Input: 一个二叉树

Output:一个每层数值组合的list

Conditions:层次遍历

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题目思路

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采用dfs，先序遍历的方法，采用一个level值来记录高度，注意要适时扩充list的size

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AC代码（Python）

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def preorder(self, root, level, res):
        if root:
            if len(res) < level + 1: res.append([])
            res[level].append(root.val)
            self.preorder(root.left, level + 1, res)
            self.preorder(root.right, level + 1, res)
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        res = []
        self.preorder(root, 0, res)
        print res
        return res