Valid Substring.cpp

/*
Valid Substring
===============

Given a string S consisting only of opening and closing parenthesis 'ie '('  and ')', find out the length of the longest valid(well-formed) parentheses substring.
NOTE: Length of smallest the valid substring ( ) is 2.

Example 1:
Input: S = "(()("
Output: 2
Explanation: The longest valid 
substring is "()". Length = 2.

Example 2:
Input: S = "()(())("
Output: 6
Explanation: The longest valid 
substring is "()(())". Length = 6.

Your Task:  
You dont need to read input or print anything. Complete the function findMaxLen() which takes S as input parameter and returns the maxlength.

Expected Time Complexity:O(n)
Expected Auxiliary Space: O(1)   

Constraints:
1 <= |S| <= 105
*/

int findMaxLen(string s)
{
  stack<int> si;
  si.push(-1);

  int ans = 0;

  for (int i = 0; i < s.size(); ++i)
  {
    if (s[i] == '(')
      si.push(i);
    else
    {
      si.pop();
      if (si.size())
      {
        int curr = i - si.top();
        ans = max(ans, curr);
      }
      else
        si.push(i);
    }
  }
  return ans;
}