[Leetcode]@python 103. Binary Tree Zigzag Level Order Traversal

题目链接

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

题目原文

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).

For example:
Given binary tree {3,9,20,#,#,15,7},

return its zigzag level order traversal as:

题目大意

"Z"字形层次遍历二叉树

解题思路

修改上一题的代码，增加对层数的判断，偶数层将元素反向插入

代码

# 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 help(self, root, level, ans):
        if root:
            if len(ans) < level + 1:
                ans.append([])
            if level % 2 == 0:
                ans[level].append(root.val)
            else:
                ans[level].insert(0, root.val)
            self.help(root.left, level + 1, ans)
            self.help(root.right, level + 1, ans)

    def zigzagLevelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        ans = []
        self.help(root, 0, ans)
        return ans