103 Binary Tree Zigzag Level Order Traversal

Original Description
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).

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• 要求： 给一个binary tree， do zigzag level traversal.

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• 返回：
  traversal 返回的数组

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• 例子：
  • input: [3,9,20,null,null,15,7] 。 3 。 /
    。 9 20 。。 /
    。。15 7
  • output: [ [3], [20,9], [15,7]]

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Solution

Referred to shenjie’s answer:

这道题跟 Binary Tree Level Order Traversal 非常相似，本质上也是树的广度优先遍历，只是在遍历的时候每一层的遍历顺序不同。那么我们只要一个变量来区分当前层是从前往后还是从后往前遍历: 当要反过来遍历节点时直接把原有的列表翻转了，而没有在生成列表的时候倒过来添加。

# 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 zigzagLevelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        
        result = []
        if not root:
            return result
        curr_level = [root]
        need_reverse = False
        while curr_level:
            level_result = []
            next_level = []
            for temp in curr_level:
                level_result.append(temp.val)
                if temp.left:
                    next_level.append(temp.left)
                if temp.right:
                    next_level.append(temp.right)
            if need_reverse:
                level_result.reverse()
                need_reverse = False
            else:
                need_reverse = True
            result.append(level_result)
            curr_level = next_level
        return result