LeetCode 102: Binary Tree Level Order Traversal — Python Solution

Solve LeetCode 102: Binary Tree Level Order Traversal in Python with a breadth-first levels 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

Process exactly the queue length captured at the start of each level and append discovered children for the next level.

Algorithm

1. Process exactly the queue length captured at the start of each level and append discovered children for the next level.
2. Maintain this invariant: The nodes removed in one batch all have the same depth.
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 levelOrder(self, root):
        if not root:
            return []
        queue, answer = deque([root]), []
        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)
        return answer

Why this is correct

The proof follows the maintained state: The nodes removed in one batch all have the same depth. 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

An empty tree yields no levels.

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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