Coder-Strike 2014 - Round 1 C. Pattern

#include <iostream>
#include <vector>
#include <algorithm>
#include <string>

using namespace std;

int main(){
int n;
cin >> n;
vector<string> patterns(n);
for(int i = 0 ; i < n; ++ i){
cin >> patterns[i];
}
string res ="";
for(int i = 0 ; i < patterns[0].length();++ i){
char ch = '?';
bool flag = false;
int j = 0;
for(j = 0; j < n; ++ j){
if(patterns[j][i] != '?' ){
flag = true;
if(ch=='?') ch = patterns[j][i] ;
else if(ch!=patterns[j][i]) {
break;
}
}
}
if(j < n) ch='?';
else if(ch=='?') ch ='a';
res+=ch;
}
cout<<res<<endl;
}