Language: C++

#include <vector>
using namespace std;
#define REP(i,x)for(int i=0;i<x;i++)

string res;
int H,W,dx,dy,min_len;
char s[200];
string MAP[11];
bool visit[20][20];
vector<string> ss;
void dfs(int x,int y,int k){
  if(visit[y][x]){
    ss.push_back(string(s));
    return;
  }
  s[k]=MAP[y][x];  visit[y][x]=1;
  dfs((x + dx + W) % W, (y + dy + H) % H, k + 1);
  s[k]=0;  visit[y][x]=0;
}
int main() {
  while(cin>>H>>W,H){
    REP(i,H)cin>>MAP[i];
    ss.clear();
    min_len=1; res="0";
    REP(y,H)REP(x,W){
      for(dx=-1;dx<=1;dx++){
        for(dy=-1;dy<=1;dy++){
          if(dx|dy)dfs(x,y,0);
        }
      }
    }
    string res = "0";
    sort(ss.begin(),ss.end());
    REP(i,ss.size()-1){
      int len = 0;
      REP(k,min(ss[i].size(),ss[i+1].size())){
        if(ss[i][k]==ss[i+1][k])len++;
        else break;
      }
      if(len < res.size())continue;
      if(res.size() == len){
        res = min(res, ss[i].substr(0,len));
      }else{
        res = ss[i].substr(0, len);
      }
    }
    cout<<res<<endl;
  }
}

Compiler output:
a.cc:5:1: error: 'string' does not name a type
    5 | string res;
      | ^~~~~~
a.cc:8:1: error: 'string' does not name a type
    8 | string MAP[11];
      | ^~~~~~
a.cc:10:8: error: 'string' was not declared in this scope
   10 | vector<string> ss;
      |        ^~~~~~
a.cc:2:1: note: 'std::string' is defined in header '<string>'; this is probably fixable by adding '#include <string>'
    1 | #include <vector>
  +++ |+#include <string>
    2 | using namespace std;
a.cc:10:14: error: template argument 1 is invalid
   10 | vector<string> ss;
      |              ^
a.cc:10:14: error: template argument 2 is invalid
a.cc: In function 'void dfs(int, int, int)':
a.cc:13:8: error: request for member 'push_back' in 'ss', which is of non-class type 'int'
   13 |     ss.push_back(string(s));
      |        ^~~~~~~~~
a.cc:13:18: error: 'string' was not declared in this scope
   13 |     ss.push_back(string(s));
      |                  ^~~~~~
a.cc:13:18: note: 'std::string' is defined in header '<string>'; this is probably fixable by adding '#include <string>'
a.cc:16:8: error: 'MAP' was not declared in this scope
   16 |   s[k]=MAP[y][x];  visit[y][x]=1;
      |        ^~~
a.cc: In function 'int main()':
a.cc:21:9: error: 'cin' was not declared in this scope
   21 |   while(cin>>H>>W,H){
      |         ^~~
a.cc:2:1: note: 'std::cin' is defined in header '<iostream>'; this is probably fixable by adding '#include <iostream>'
    1 | #include <vector>
  +++ |+#include <iostream>
    2 | using namespace std;
a.cc:22:18: error: 'MAP' was not declared in this scope
   22 |     REP(i,H)cin>>MAP[i];
      |                  ^~~
a.cc:23:8: error: request for member 'clear' in 'ss', which is of non-class type 'int'
   23 |     ss.clear();
      |        ^~~~~
a.cc:24:16: error: 'res' was not declared in this scope
   24 |     min_len=1; res="0";
      |                ^~~
a.cc:32:5: error: 'string' was not declared in this scope
   32 |     string res = "0";
      |     ^~~~~~
a.cc:32:5: note: 'std::string' is defined in header '<string>'; this is probably fixable by adding '#include <string>'
a.cc:33:13: error: request for member 'begin' in 'ss', which is of non-class type 'int'
   33 |     sort(ss.begin(),ss.end());
      |             ^~~~~
a.cc:33:24: error: request for member 'end' in 'ss', which is of non-class type 'int'
   33 |     sort(ss.begin(),ss.end());
      |                        ^~~
a.cc:33:5: error: 'sort' was not declared in this scope; did you mean 'short'?
   33 |     sort(ss.begin(),ss.end());
      |     ^~~~
      |     short
a.cc:34:14: error: request for member 'size' in 'ss', which is of non-class type 'int'
   34 |     REP(i,ss.size()-1){
      |              ^~~~
a.cc:3:31: note: in definition of macro 'REP'
    3 | #define REP(i,x)for(int i=0;i<x;i++)
      |                               ^
a.cc:36:19: error: invalid types 'int[int]' for array subscript
   36 |       REP(k,min(ss[i].size(),ss[i+1].size())){
      |                   ^
a.cc:3:31: note: in definition of macro 'REP'
    3 | #define REP(i,x)for(int i=0;i<x;i++)
      |                               ^
a.cc:36:32: error: invalid types 'int[int]' for array subscript
   36 |       REP(k,min(ss[i].size(),ss[i+1].size())){
      |                                ^
a.cc:3:31: note: in definition of macro 'REP'
    3 | #define REP(i,x)for(int i=0;i<x;i++)
      |                               ^
a.cc:37:14: error: invalid types 'int[int]' for array subscript
   37 |         if(ss[i][k]==ss[i+1][k])len++;
      |              ^
a.cc:37:24: error: invalid types 'int[int]' for array subscript
   37 |         if(ss[i][k]==ss[i+1][k])len++;
      |                        ^
a.cc:42:26: error: invalid types 'int[int]' for array subscript
   42 |         res = min(res, ss[i].substr(0,len));
      |                          ^
a.cc:44:17: error: invalid types 'int[int]' for array subscript
   44 |         res = ss[i].substr(0, len);
      |                 ^
a.cc:47:5: error: 'cout' was not declared in this scope
   47 |     cout<<res<<endl;
      |     ^~~~
a.cc:47:5: note: 'std::cout' is defined in header '<iostream>'; this is probably fixable by adding '#include <iostream>'
a.cc:47:16: error: 'endl' was not declared in this scope
   47 |     cout<<res<<endl;
      |                ^~~~
a.cc:2:1: note: 'std::endl' is defined in header '<ostream>'; this is probably fixable by adding '#include <ostream>'
    1 | #include <vector>
  +++ |+#include <ostream>
    2 | using namespace std;