## C# || Longest Valid Parentheses – How To Find The Longest Valid Well Formed Parentheses Using C#

The following is a module with functions which demonstrates how to find the longest valid well formed parentheses using C#.

1. Longest Valid Parentheses – Problem Statement

Given a string containing just the characters **‘(‘** and **‘)’**, find the length of the longest valid (well-formed) parentheses substring.

**Example 1:**

Input:s = "(()"

Output:2

Explanation:The longest valid parentheses substring is "()".

**Example 2:**

Input:s = ")()())"

Output:4

Explanation:The longest valid parentheses substring is "()()".

**Example 3:**

Input:s = ""

Output:0

2. Longest Valid Parentheses – Solution

The following is a solution which demonstrates how to find the longest valid well formed parentheses.

In this solution we can make use of a stack while scanning the given string to:

- Check if the string scanned so far is valid
- Find the length of the longest valid string

In order to do so, we start by pushing **-1** onto the stack. For every ‘**(**‘ encountered, we push its index onto the stack.

For every ‘**)**‘ encountered, we pop the topmost element. Then, the length of the currently encountered valid string of parentheses will be the difference between the current element’s index and the top element of the stack.

If, while popping the element, the stack becomes empty, we will push the current element’s index onto the stack. In this way, we can continue to calculate the length of the valid substrings and return the length of the longest valid string at the end.

```
```
123456789101112131415161718192021222324252627
// ============================================================================// Author: Kenneth Perkins// Date: May 24, 2022// Taken From: http://programmingnotes.org/// File: Solution.cs// Description: Demonstrates how to find the longest valid parentheses// ============================================================================public class Solution { public int LongestValidParentheses(string s) { var result = 0; var stack = new Stack<int>(); stack.Push(-1); for (var index = 0; index < s.Length; ++index) { if (s[index] == '(') { stack.Push(index); } else { stack.Pop(); if (stack.Count == 0) { stack.Push(index); } else { result = Math.Max(result, index - stack.Peek()); } } } return result; }}// http://programmingnotes.org/

**QUICK NOTES**:

The highlighted lines are sections of interest to look out for.

The code is heavily commented, so no further insight is necessary. If you have any questions, feel free to leave a comment below.

Once compiled, you should get this as your output for the example cases:

2

4

0

## Leave a Reply