dp方法：

class Solution {
public:
    int longestValidParentheses(string s) {
        int n=s.size();
        vector<int> dp(n,0);
        if(n==0 || n==1) return 0;
        if(s[0]=='(' && s[1]==')'){
            dp[1]=2;
        }
        for(int i=2;i<n;i++){
            if(s[i]==')'){
                if(s[i-1]=='('){
                    dp[i]=dp[i-2]+2;
                }else if(s[i-1]==')'){
                    if(i-dp[i-1]-1>=0 && s[i-dp[i-1]-1]=='('){
                        if(i-dp[i-1]-2>=0)
                        dp[i]=dp[i-1]+2+dp[i-dp[i-1]-2];
                        else 
                        dp[i]=dp[i-1]+2;
                    }
                }
            }
        }
        return *max_element(dp.begin(), dp.end());
    }
};

连续1方法：

class Solution {
public:
    int longestValidParentheses(string s) {
        int n=s.size();
        vector<pair<char,int>> stack;
        vector<int> tmp(n,0);
        for(int i=0;i<n;i++){
            if(stack.empty()){
                stack.push_back(make_pair(s[i],i));
            }
            else{
                if(stack.back().first=='(' && s[i]==')'){
                    tmp[i]=tmp[stack.back().second]=1;  
                    stack.pop_back();
                }else {
                    stack.push_back(make_pair(s[i],i));
                }
            }
        }
        int count=0;
        int count_max=0;
        for(int i=0;i<n;i++){
            if(tmp[i]==1){
                count++;
            }else{
                count_max=max(count_max,count);
                count=0;
            }
        }
        count_max=max(count_max,count);
        return count_max;
    }
};