LeetCode 20: Valid Parentheses — Python Solution

Solve LeetCode 20: Valid Parentheses in Python with a expected-closer stack 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

Push the required closing symbol for every opener; each closer must match the most recently expected symbol.

Algorithm

1. Push the required closing symbol for every opener; each closer must match the most recently expected symbol.
2. Maintain this invariant: The stack contains exactly the closing symbols needed for unmatched openers.
3. Continue until every input item or reachable state has been resolved, then return the accumulated result.

Python solution

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

class Solution:
    def isValid(self, s):
        expected = []
        pairs = {"(": ")", "[": "]", "{": "}"}
        for char in s:
            if char in pairs:
                expected.append(pairs[char])
            elif not expected or expected.pop() != char:
                return False
        return not expected

Why this is correct

The proof follows the maintained state: The stack contains exactly the closing symbols needed for unmatched openers. 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(n) space. The stated auxiliary space excludes the returned output unless the output is the data structure being built.

Edge cases

A closer on an empty stack or leftover openers makes the string invalid.

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.

Browse the searchable 100 LeetCode Python Solutions hub. Previous: 452. Minimum Number of Arrows to Burst Balloons · Next: 71. Simplify Path