103. Binary Tree Zigzag Level Order Traversal

Medium LeetCode

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

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

# 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)