LeetCode 103. Binary Tree Zigzag Level Order Traversal

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题目

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],

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2
3
4
5
  3
 / \
9  20
  /  \
 15   7

return its zigzag level order traversal as:

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2
3
4
5
[
  [3],
  [20,9],
  [15,7]
]

思路

Queue.

代码

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# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution(object):
    def zigzagLevelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        queue = collections.deque([root])
        res = []
        while queue:
            r = []
            for _ in range(len(queue)):
                q = queue.popleft()
                if q:
                    r.append(q.val)
                    queue.append(q.left)
                    queue.append(q.right)
                    
            r = r[::-1] if len(res) % 2 else r
            if r:
                res.append(r)
        return res