// Longest Valid Parentheses — HARD

// Category: stack

Given a string containing just the characters `'('` and `')'`, return the length of the longest valid (well-formed) parentheses substring.

Hint: Use a stack. Push indices of unmatched `'('`. When you see `')'`, pop from the stack — if empty, push the current index as the new base; otherwise, the current valid length is `i - stack.top()`.

Example: s = "(()"

Output: 2