How hard is LeetCode 103. Binary Tree Zigzag Level Order Traversal?

LeetCode 103. Binary Tree Zigzag Level Order Traversal is rated Medium on LeetCode.

What is the time complexity of LeetCode 103. Binary Tree Zigzag Level Order Traversal?

The Python solution on this page runs in O(n).

What is the space complexity of LeetCode 103. Binary Tree Zigzag Level Order Traversal?

The Python solution on this page uses O(n) auxiliary space.

What topics does LeetCode 103. Binary Tree Zigzag Level Order Traversal cover?

LeetCode 103. Binary Tree Zigzag Level Order Traversal is tagged Tree, Breadth-First Search and Binary Tree on LeetCode.

Binary Tree Zigzag Level Order Traversal — LeetCode 103 Python Solution

MediumTreeBreadth-First SearchBinary Tree

Problem: #103
Pattern: Tree Traversal
Reading time: 5 min
Source: leetcode.com

The problem

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

Input: root = [3,9,20,null,null,15,7]
Output: [[3],[20,9],[15,7]]

Python solution

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]]:
        ans = []
        if root is None:
            return ans
        q = deque([root])
        ans = []
        left = 1
        while q:
            t = []
            for _ in range(len(q)):
                node = q.popleft()
                t.append(node.val)
                if node.left:
                    q.append(node.left)
                if node.right:
                    q.append(node.right)
            ans.append(t if left else t[::-1])
            left ^= 1
        return ans

Complexity

Measure	Complexity
Time	O(n)
Space	O(n) auxiliary

Pattern: Tree Traversal

Choose the order — preorder, inorder, postorder, level — and the problem solves itself. LeetCode 103. Binary Tree Zigzag Level Order Traversal is filed here because LeetCode tags it Tree and Binary Tree, which is the vocabulary this hub collects.

The tree traversal guide has the Python template for the pattern and the 225 LeetCode problems that use it.

On a study list

This problem is on Top Interview 150.

Frequently asked questions

How hard is LeetCode 103. Binary Tree Zigzag Level Order Traversal?: LeetCode 103. Binary Tree Zigzag Level Order Traversal is rated Medium on LeetCode.
What is the time complexity of LeetCode 103. Binary Tree Zigzag Level Order Traversal?: The Python solution on this page runs in O(n).
What is the space complexity of LeetCode 103. Binary Tree Zigzag Level Order Traversal?: The Python solution on this page uses O(n) auxiliary space.
What topics does LeetCode 103. Binary Tree Zigzag Level Order Traversal cover?: LeetCode 103. Binary Tree Zigzag Level Order Traversal is tagged Tree, Breadth-First Search and Binary Tree on LeetCode.