Leetcode 103. Binary Tree Zigzag Level Order Traversal
https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal/
Medium
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] ]
- 类似BFS,一层层扩展。先从左到右把node放到list里,再判断是否需要反转。
1 # Definition for a binary tree node. 2 # class TreeNode: 3 # def __init__(self, x): 4 # self.val = x 5 # self.left = None 6 # self.right = None 7 8 class Solution: 9 def zigzagLevelOrder(self, root: TreeNode) -> List[List[int]]: 10 if root is None: 11 return [] 12 13 res = [] 14 cur = [root] 15 from_right = False 16 17 while cur: 18 next_level, vals = [], [] 19 20 for node in cur: 21 vals.append(node.val) 22 23 if node.left: 24 next_level.append(node.left) 25 26 if node.right: 27 next_level.append(node.right) 28 29 res.append(vals[::-1] if from_right else vals) # trick 30 from_right = not from_right 31 32 cur = next_level 33 34 return res