Longest Valid Parentheses

int longestValidParentheses(string s) {

vector<int> f(s.size(),0);

int ret=0;

for(int i=s.size()-2;i>=0;i--){

int match = i+f[i+1]+1;

if(s[i]=='(' && match<s.size() && s[match]==')'){

f[i]=f[i+1]+2;

if(match+1<s.size()) f[i]+=f[match+1];

}

ret=max(ret,f[i]);

}

return ret;

}