LeetCode 103 Binary Tree Zigzag Level Order Traversal

LeetCode

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

Example 1:

<pre>Input: root = [3,9,20,null,null,15,7] Output: [[3],[20,9],[15,7]] </pre>

Example 2:

<pre>Input: root = [1] Output: [[1]] </pre>

Example 3:

<pre>Input: root = [] Output: [] </pre>

Constraints:

• The number of nodes in the tree is in the range [0, 2000].
• -100 <= Node.val <= 100

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题目大意：

按层遍历二叉树。偶数层逆向

解题思路：

用BFS按层遍历模板

解题步骤：

N/A

注意事项：

1. 多这一行level.append(node.val)

Python代码：

def zigzagLevelOrder(self, root: TreeNode) -> List[List[int]]:
	if not root:
		return []
	res = []
	queue = collections.deque([root])
	while queue:
		level = []
		for _ in range(len(queue)):
			node = queue.popleft()
			level.append(node.val)
			if node.left:
				queue.append(node.left)
			if node.right:
				queue.append(node.right)
		if len(res) % 2 == 1:
			res.append(level[::-1])
		else:
			res.append(level)
	return res

算法分析：

时间复杂度为O(n)，空间复杂度O(1)