leetcode 32 Longest Valid Parentheses
class Solution { public: int longestValidParentheses(string s) { int res = 0, start = 0; stack<int> m; for (int i = 0; i < s.size(); ++i) { if (s[i] == '(') m.push(i); else if (s[i] == ')') { if (m.empty()) start = i + 1; else { m.pop(); res = m.empty() ? max(res, i - start + 1) : max(res, i - m.top()); } } } return res; } };