leetcode Longest Valid Parentheses

 

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

For "(()", the longest valid parentheses substring is "()", which has length = 2.

Another example is ")()())", where the longest valid parentheses substring is "()()", which has length = 4.

 

The stack has to record the position before matching '('. If matching, The position of ')' - the position + 1 is the answer since the parentheses between them  have been eliminated. 

 

 

class Solution {
 public:
  int longestValidParentheses(string s) {
    // Note: The Solution object is instantiated only once and is reused by each test case.
    stack<int> sta;
    int i = 0, j = 0, maxV = 0, count = 0;
    for (i = 0; i < s.length(); ++i) {
      if (s[i] == '(')
        sta.push(i);
      else {
        if (!sta.empty() && s[sta.top()] == '(') {
          sta.pop();
          if (sta.empty())
            count = i + 1;
          else
            count = i - sta.top();
          if (count > maxV)
            maxV = count;
        }
        else
          sta.push(i);
      }
    }
    return maxV;
  }
};

Python Version:

class Solution:
    def longestValidParentheses(self, s):
        maxCount, slen = 0, len(s)
        stack = []
        for i in range(slen):
            if (s[i] == '('):
                stack.append(i)
            elif (s[i] == ')'):
                if (stack and s[stack[-1]] == '('):
                    stack.pop()
                    lastPos = stack[-1]+1 if stack else 0
                    curCount = i-lastPos+1
                    if (curCount > maxCount):
                        maxCount = curCount
                else:
                    stack.append(i)
        return maxCount
s = Solution()
print(s.longestValidParentheses(")()())"))