Pai nu e asa de greu :), diagonala principala are proprietatea ca i=j=numarul caracterului.
#include <bits/stdc++.h>
using namespace std;
int n,k;
char A[11][11],s[11];
int main()
{
cin >> n;
for(int i=1;i<=n;i++)
for(int j=1;j<=n;j++)
cin >> A[i][j];
for(int i=1;i<=n;i++)
for(int j=1;j<=n;j++)
if(j==i) s[k++]=A[i][j];
cout << s;
return 0;
}