LeetCode 199: Binary Tree Right Side View — Python Solution

Solve LeetCode 199: Binary Tree Right Side View in Python with a level-order last node 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 the tree level by level and record the value of the final node dequeued from each level.

Algorithm

1. Process the tree level by level and record the value of the final node dequeued from each level.
2. Maintain this invariant: The queue segment for a level is ordered left-to-right, so its final node is visible from the right.
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 rightSideView(self, root):
        if not root:
            return []
        queue, answer = deque([root]), []
        while queue:
            for _ in range(len(queue)):
                node = queue.popleft()
                if node.left: queue.append(node.left)
                if node.right: queue.append(node.right)
            answer.append(node.val)
        return answer

Why this is correct

The proof follows the maintained state: The queue segment for a level is ordered left-to-right, so its final node is visible from the right. 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 produces an empty list.

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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