string Solution::longestCommonPrefix(vector<string> &A) {
if(A.size()==1) return A[0];
string sout = "";
sort(A.begin(),A.end());
auto it = A.rbegin();
int l= 1;
string s1 = A[0];
string s2 = *it;
//cout<<s1<<" "<<s2<<endl;
int i=0;
while(1)
{
if(s1[i] == s2[i]) {sout+=s1[i];i++;}
else break;
}
return sout;
}