Longest Common Prefix

#1

my code is giving run time error on leetcode

#2
#3 
 int n1  = prefix.length();
    int n2  = s.length();
    string res="";

    for (int i=0;i<min(prefix.size(),s.size());i++)
    {
        if (prefix[i] != s[i])
        {
            break;
        }

        else
            res+=prefix[i];
    }
    return res;