103. Binary Tree Zigzag Level Order Traversal

• categories: Code, Interview Question, leetcode, Medium
• source: https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal
• topics: Tree, BFS

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).

Idea

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 not root:
            return []
        ans = []
        d = deque([root])
        direction = 1
        while d:
            level_nodes = []
            for _ in range(len(d)):
                element = d.popleft()
                if direction:
                    level_nodes.append(element.val)
                else:
                    level_nodes.insert(0, element.val)
                if element.left:
                    d.append(element.left)
                if element.right:
                    d.append(element.right)
            
            ans.append(level_nodes)
            direction = (direction + 1) % 2
            
        return ans