Valid Parentheses In Python: A Comprehensive Guide

Nov 16, 2025 by Alex Johnson 51 views

Have you ever struggled with validating parentheses in a string? It's a common problem in computer science, often encountered in coding interviews and practical applications. This article will delve into the concept of valid parentheses, providing a clear explanation and a Python solution. We'll break down the problem, explore the underlying logic, and walk through the code step-by-step, ensuring you grasp the fundamentals and can confidently tackle similar challenges.

Understanding the Problem: What are Valid Parentheses?

Before diving into the code, let's define what constitutes valid parentheses. In essence, a string of parentheses is considered valid if:

Each opening parenthesis has a corresponding closing parenthesis.
The parentheses are closed in the correct order.

For instance, "()", "()[]{}", and "{[]}" are valid, while "(]", "([)]", and ")(" are not. The key is to ensure that for every closing parenthesis, there's a matching opening parenthesis of the same type that hasn't been closed yet, and that the order of closure is correct. This problem often arises in parsing expressions, validating code syntax, and other areas where structured data needs to be analyzed. A solid understanding of this concept is crucial for any aspiring programmer or software developer.

The Logic Behind the Solution: Leveraging the Stack Data Structure

To solve this problem efficiently, we can employ a stack data structure. A stack operates on the principle of Last-In, First-Out (LIFO). Think of it like a stack of plates: the last plate you put on top is the first one you take off. This LIFO behavior is perfectly suited for our task.

Here's the core idea:

Iterate through the input string character by character.
If we encounter an opening parenthesis ('(', '{', or '['), we push it onto the stack.
If we encounter a closing parenthesis (')', '}', or ']'), we check:
- If the stack is empty, it means there's no corresponding opening parenthesis, so the string is invalid.
- If the stack is not empty, we pop the top element from the stack and check if it matches the closing parenthesis. If they don't match, the string is invalid.
After processing the entire string, if the stack is empty, it means all opening parentheses have been matched with their corresponding closing parentheses, and the string is valid. If the stack is not empty, it means there are unmatched opening parentheses, and the string is invalid.

The beauty of using a stack lies in its ability to keep track of the order in which opening parentheses appear. By pushing opening parentheses onto the stack and popping them when we encounter closing parentheses, we ensure that the parentheses are closed in the correct order. This approach provides a clean and efficient way to validate parentheses.

Python Code Implementation: A Step-by-Step Walkthrough

Now, let's translate this logic into Python code. Here's the Python solution for validating parentheses:

class Solution(object):
    def isValid(self, s):
        """
        :type s: str
        :rtype: bool
        """
        stack = []
        mapping = {')': '(', '}': '{', ']': '['}

        for char in s:
            if char in mapping:
                # Pop from stack if available, else assign a dummy value
                top_element = stack.pop() if stack else '#'
                if mapping[char] != top_element:
                    return False
            else:
                stack.append(char)

        return not stack

Let's break down this code snippet:

class Solution(object):: This defines a class named Solution, which is a common practice in coding platforms like LeetCode. It's a way to encapsulate the solution within a class structure.
def isValid(self, s):: This defines a method named isValid within the Solution class. This method takes a string s as input and returns a boolean value indicating whether the parentheses in the string are valid.
stack = []: This initializes an empty list called stack. This list will serve as our stack data structure.
mapping = {')': '(', '}': '{', ']': '['}: This creates a dictionary called mapping. This dictionary maps each closing parenthesis to its corresponding opening parenthesis. This mapping makes it easy to check if a closing parenthesis matches the top element of the stack.
for char in s:: This loop iterates through each character char in the input string s.
if char in mapping:: This condition checks if the current character char is a closing parenthesis. We do this by checking if the character is a key in the mapping dictionary.
top_element = stack.pop() if stack else '#': If the character is a closing parenthesis, this line attempts to pop the top element from the stack. If the stack is empty (meaning there's no corresponding opening parenthesis), it assigns a dummy value '#' to top_element. This prevents an error from occurring when trying to pop from an empty stack.
if mapping[char] != top_element:: This checks if the opening parenthesis that corresponds to the current closing parenthesis (obtained from the mapping dictionary) matches the top_element popped from the stack. If they don't match, it means the parentheses are not valid, so we immediately return False.
else:: If the character is not a closing parenthesis (meaning it's an opening parenthesis), this block is executed.
stack.append(char): This line pushes the opening parenthesis char onto the stack.
return not stack: After processing the entire string, this line returns True if the stack is empty (meaning all opening parentheses have been matched) and False otherwise (meaning there are unmatched opening parentheses).

This code effectively implements the stack-based logic we discussed earlier, providing a clear and concise solution to the valid parentheses problem. By understanding each step, you can confidently apply this solution to various scenarios.

Examples and Test Cases: Putting the Code to the Test

To ensure our solution works correctly, let's test it with some examples:

Input: `s =