求最长的满足括号关系的对应子串。栈记录即可。

class Solution {
public:
    int longestValidParentheses(string s) {
        stack<int>st;
        st.push(-1);
        int max = 0;
        int i = 0;
        while(i < s.length()) {
            int top = st.top();
            if(top == -1) {
                st.push(i);
            }
            else {
                if(s[top] == '(' && s[i] == ')') {
                    st.pop();
                }
                else {
                    st.push(i);
                }
            }
            i++;
        }
        int now = s.length();
        while(!st.empty()) {
            int before = st.top();
            st.pop();
            if((now - 1 - before) % 2 == 0 && now - 1 - before > max)
                max = now - 1 - before;
            now = before;
        }
        return max;
    }
};