LeetCode 32. Longest Valid Parentheses

#include <string>
#include <vector>
#include <iostream>
using namespace std;

/*
  Given a string containing just the character '(' and ')', find the length of the longest
  valid parentheses substring.
  for "(()", the longest valid parentheses substring is  "()" which has length 2.
  Another example ")()())", where the longest valid parentheses substring is "()()", which 
  has length 4.
*/

// Use one pointer always points to the end index of vector.

int longestValidParentheses(string s) {
  if(s.size() <= 1) return 0;
  vector<int> index;
  int maxSize = 0;
  for(int i = 0; i < s.size(); ++i) {
    if(s[i] == '(') {
      index.push_back(i);   // push the index number.
    } else {
      if(!index.empty() && s[index.back()] == '(') {
        index.pop_back();
        int lastPos = -1;
        if(!index.empty()) {
            lastPos = index.back();
        }
        maxSize = max(maxSize, i - lastPos);
      } else {
        index.push_back(i);
      }
    }
  }
    return maxSize;
}

int main(void) {
  cout << longestValidParentheses("())()") << endl;
}
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