LeetCode 103: Binary Tree Zigzag Level Order Traversal — Python Solution

Solve LeetCode 103: Binary Tree Zigzag Level Order Traversal in Python with a alternating level output approach. The key is to make the state invariant explicit, so the implementation and complexity follow naturally.

This guide paraphrases the task and does not reproduce LeetCode’s prompt. Use the official page for the complete statement, examples, constraints, and submission runner.

What the problem is testing

Run ordinary BFS and reverse the collected values on alternating levels.

Algorithm

1. Run ordinary BFS and reverse the collected values on alternating levels.
2. Maintain this invariant: Each level is collected left-to-right before the direction flag determines output order.
3. Continue until every input item or reachable state has been resolved, then return the accumulated result.

Python solution

LeetCode provides the list, tree, or graph node definition used by the method.

from collections import Counter, defaultdict, deque, OrderedDict
import random

class Solution:
    def zigzagLevelOrder(self, root):
        if not root:
            return []
        queue, answer, reverse = deque([root]), [], False
        while queue:
            level = []
            for _ in range(len(queue)):
                node = queue.popleft(); level.append(node.val)
                if node.left: queue.append(node.left)
                if node.right: queue.append(node.right)
            answer.append(level[::-1] if reverse else level)
            reverse = not reverse
        return answer

Why this is correct

The proof follows the maintained state: Each level is collected left-to-right before the direction flag determines output order. Each iteration preserves that claim while permanently resolving at least one position, node, interval, or search state. When the loop or recursion ends, every candidate required by the problem has therefore been included or ruled out, so the returned value is correct.

Complexity

O(n) time and O(w) space. The stated auxiliary space excludes the returned output unless the output is the data structure being built.

Edge cases

The root level is left-to-right; direction flips after every level.

Tested reference code

This implementation is included in the site’s downloadable 100-solution Python library. The complete suite compiles every solution and runs a behavioral assertion for every problem before publication.

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