最长有效括号

先用栈判断括号匹配，在把所有匹配的括号标记为0，然后求最长的0序列

  class Solution {
  public:
      int longestValidParentheses(string s) {
          int len = s.length();
          int ans = 0;
          stack<int>p;
          for (int i = 0; i < len; i++) {
              if (s[i] == '(') {
                  p.push(i);
              }
              if (s[i] == ')' && !p.empty()) {
                  s[i] = '0';
                  s[p.top()] = '0';
                  p.pop();
              }
         }
          int temp = 0;
          for (int i = 0; i < len; i++) {
              if (s[i] == '0') {
                  temp++;
              }
              else {
                  ans = max(ans, temp);
                  temp = 0;
              }
         }
          ans = max(ans, temp);
          return ans;
      }
  };