leetcode - Longest Valid Parentheses

题目描述：点击此处

class Solution {
public:
  int longestValidParentheses(string s) {
    // Start typing your C/C++ solution below
    // DO NOT write int main() function
    stack<int> lefts;
    int len = s.length();
    int * f = new int [len];
    int rlt = 0;
    int i ;
    for (i=0; i<len; i++)
      f[i] = -1;
    for (i=0; i<len; i++){
      char c = s.at(i);
      if (c=='('){
        lefts.push(i);
      }
      else if (!lefts.empty()){
        int t = lefts.top();
        lefts.pop();
        f[t] = (i-t+1)/2;
      }
    }
    i = 0;
    while (i<len){
      if (f[i] == -1){
        i++;
        continue;
      }
      int tlen = 0;
      while (i<len && f[i]!=-1){
        tlen += f[i];
        if (tlen > rlt)
          rlt = tlen ;
        i += 2*f[i];
      }
    }
    delete []f;
    return 2  * rlt;
  }
};