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LeetCode 103 Binary Tree Zigzag Level Order Traversal - Medium

103. Binary Tree Zigzag Level Order Traversal

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103. Binary Tree Zigzag Level Order Traversal — Medium

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Problem

  1. Binary Tree Zigzag Level Order Traversal

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,null,null,15,7], 3 /
9 20 /
15 7 return its zigzag level order traversal as: [ [3], [20,9], [15,7] ]

Solution

# 先顺序放，按level最后翻转比较方便。
 
# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def zigzagLevelOrder(self, root: TreeNode) -> List[List[int]]:
        if not root:
            return []
        from collections import deque
        stack = deque()
        stack.append(root)
        count = 0
        res = []
        while stack:
            temp = []
            l = len(stack)
            for _ in range(l):
                node = stack.popleft()
                temp.append(node.val)
                if node.right:
                    stack.append(node.right)
                if node.left:
                    stack.append(node.left)
            if count % 2 == 0:
                temp = temp[::-1]
            count += 1
            res.append(temp)
        
        return res

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