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32\. Longest Valid Parentheses

Hard

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 

**Constraints:**

*   0 <= s.length <= 3 * 10⁴
*   `s[i]` is `'('`, or `')'`.

To solve the "Longest Valid Parentheses" problem in Java with a `Solution` class, we can follow these steps:

1. Define a `Solution` class.
2. Define a method named `longestValidParentheses` that takes a string `s` as input and returns an integer representing the length of the longest valid parentheses substring.
3. Initialize a stack to store the indices of characters.
4. Initialize a variable `maxLen` to store the maximum length of valid parentheses found so far.
5. Push `-1` onto the stack to mark the starting point of a potential valid substring.
6. Iterate through each character of the string:
   - If the character is `'('`, push its index onto the stack.
   - If the character is `')'`:
     - Pop the top index from the stack.
     - If the stack is empty after popping, push the current index onto the stack to mark the starting point of the next potential valid substring.
     - Otherwise, update `maxLen` with the maximum of the current `maxLen` and `i - stack.peek()`, where `i` is the current index and `stack.peek()` is the index at the top of the stack.
7. Return `maxLen`.

Here's the implementation:

```java
import java.util.Stack;

public class Solution {
    public int longestValidParentheses(String s) {
        int maxLen = 0;
        Stack stack = new Stack<>();
        stack.push(-1); // Mark the starting point of a potential valid substring
        
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (c == '(') {
                stack.push(i);
            } else { // c == ')'
                stack.pop();
                if (stack.isEmpty()) {
                    stack.push(i); // Mark the starting point of the next potential valid substring
                } else {
                    maxLen = Math.max(maxLen, i - stack.peek());
                }
            }
        }
        
        return maxLen;
    }
}
```

This implementation provides a solution to the "Longest Valid Parentheses" problem in Java. It finds the length of the longest valid parentheses substring in the given string `s`.