103. Binary Tree Zigzag Level Order Traversal
Medium LeetCodeGiven 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
Input: root = [3,9,20,null,null,15,7]
Output: [[3],[20,9],[15,7]]
Example 2
Input: root = [1]
Output: [[1]]
Example 3
Input: root = []
Output: []
Constraints
- The number of nodes in the tree is in the range [0, 2000]
- -100 <= Node.val <= 100
How to solve the problem
Code
- BFS
Python
# 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: Optional[TreeNode]) -> List[List[int]]:
if root is None:
return []
ans = []
cur = [root]
even = False
while cur:
nxt = []
vals = []
for node in cur:
vals.append(node.val)
if node.left:
nxt.append(node.left)
if node.right:
nxt.append(node.right)
cur = nxt
ans.append(vals[::-1] if even else vals) # Reverse if even
even = not even
return ans
Complexity
- Time complexity: O(n), n == number of nodes
- Space complexity: O(n)
Comments
No comments yet. Be the first to comment!