Find longest prefix among strings

#include <iostream>
#include <string>
#include <vector>
using namespace std;

string longestCommonPrefix(vector<string> &A) {
    int in=A[0].length()-1;
   // cout<<in;
    for(int i=0;i<A.size()-1;i++)
    {
        cout<<in<<endl;
        while(A[i][in]!=A[i+1][in]) {
            cout<<"equal";
          in--;
          }
    }
    string str;
    str=A[0].substr(0,in+1);
    return str;
    }
int main()
{
    vector <string> v;
    v.push_back("hello");
    v.push_back("helpp");
    v.push_back("heloo");
    string str=longestCommonPrefix(v);
    cout<<str;
}