Mastering the Stack Data Structure with Valid Parentheses

Today, we will explore the Stack data structure through a classic Leetcode problem known as Valid Parentheses. This problem is a foundational exercise that helps you understand how to manage data in a last-in, first-out (LIFO) manner, which is the core principle of stacks.

Prerequisites

Before we dive into the problem, it’s helpful to have a basic understanding of the following concepts:

• Data Structures: Familiarity with basic data structures like arrays and lists.
• Algorithms: A general understanding of how algorithms work, especially those that involve iteration and recursion.
• Programming Language: Basic knowledge of a programming language such as Python, Java, or JavaScript.

Understanding the Problem

The Valid Parentheses problem asks you to determine if a string containing just the characters (, ), {, }, [, and ] is valid. A string is considered valid if:

1. Every opening bracket has a corresponding closing bracket.
2. Brackets are closed in the correct order.

For example:

• () is valid.
• ()[]{} is valid.
• (] is not valid.
• ([)] is not valid.

Step-by-Step Guide to Solve the Problem

Now that we understand the problem, let’s walk through how to solve it using a stack.

Step 1: Initialize a Stack

We will use a stack to keep track of the opening brackets. When we encounter a closing bracket, we will check if it matches the top of the stack.

stack = []

Step 2: Iterate Through the String

Next, we will loop through each character in the string:

for char in s:

Step 3: Handle Opening Brackets

If the character is an opening bracket, we will push it onto the stack:

if char in "({[":
    stack.append(char)

Step 4: Handle Closing Brackets

If the character is a closing bracket, we will check if the stack is not empty and if the top of the stack matches the corresponding opening bracket:

elif char in ")}]":
    if not stack or not is_matching(stack.pop(), char):
        return False

Step 5: Check for Remaining Brackets

Finally, after iterating through the string, we need to check if the stack is empty. If it is empty, the string is valid; otherwise, it is not:

return len(stack) == 0

Explanation of the Code

Let’s break down the code we just wrote:

• The stack is used to store opening brackets as we encounter them.
• When we find a closing bracket, we check if it matches the last opened bracket by popping from the stack.
• If at any point we find a mismatch or if the stack is empty when it shouldn’t be, we return False.
• At the end, if the stack is empty, it means all brackets were matched correctly, and we return True.

Conclusion

In this tutorial, we explored the Stack data structure through the Valid Parentheses problem. By using a stack, we were able to efficiently determine if a string of brackets is valid. This exercise not only helps in understanding stacks but also prepares you for more complex problems in data structures and algorithms.

For further practice, you can try solving similar problems on platforms like Leetcode or HackerRank. Happy coding!

For more details, check out the original problem description here: Continue reading on Medium ».