Language: C++

#include <cstdio>
#include <cstring>
#include <string>
#include <iostream>
#include <map>
#include <vector>
#include <algorithm>
using namespace std;

int h,w;
string fie[11];
int dx[8]={1,0,-1,0,1,1,-1,-1};
int dy[8]={0,1,0,-1,1,-1,1,-1};
map<string,int> cnt;
bool used[11][21];
vector<string> index;

void dfs(int y,int x,int k,string str){
	if(used[y][x])return;
	used[y][x]=true;
	str+=fie[y][x];
	cnt[str]++;
	index.push_back(str);
	dfs((y+dy[k]+h)%h,(x+dx[k]+w)%w,k,str);
}

int main(void){
	while(1){
		scanf("%d %d",&h,&w);
		if(h+w==0)break;
		cnt.clear();
		index.clear();
		for(int i=0;i<h;i++){
			cin >> fie[i];
		}
		for(int i=0;i<h;i++){
			for(int j=0;j<w;j++){
				for(int k=0;k<8;k++){
					memset(used,false,sizeof(used));
					dfs(i,j,k,"");
				}
			}
		}
		sort(index.begin(),index.end());
		index.erase(unique(index.begin(),index.end()),index.end());
		string res="0";
		for(int i=0;i<index.size();i++){
			if(cnt[index[i]]>=2 && index[i].size()>=2){
				if(res.size()<index[i].size() || (res.size()==index[i].size() && index[i]<res)){
					res=index[i];
				}
			}
		}
		cout << res << endl;
	}
	return 0;
}

Compiler output:
a.cc:16:16: error: 'std::vector<std::__cxx11::basic_string<char> > index' redeclared as different kind of entity
   16 | vector<string> index;
      |                ^~~~~
In file included from /usr/include/string.h:462,
                 from /usr/include/c++/14/cstring:43,
                 from a.cc:2:
/usr/include/strings.h:50:20: note: previous declaration 'const char* index(const char*, int)'
   50 | extern const char *index (const char *__s, int __c)
      |                    ^~~~~
a.cc: In function 'void dfs(int, int, int, std::string)':
a.cc:23:15: error: overloaded function with no contextual type information
   23 |         index.push_back(str);
      |               ^~~~~~~~~
a.cc: In function 'int main()':
a.cc:32:23: error: overloaded function with no contextual type information
   32 |                 index.clear();
      |                       ^~~~~
a.cc:44:28: error: overloaded function with no contextual type information
   44 |                 sort(index.begin(),index.end());
      |                            ^~~~~
a.cc:44:42: error: overloaded function with no contextual type information
   44 |                 sort(index.begin(),index.end());
      |                                          ^~~
a.cc:45:23: error: overloaded function with no contextual type information
   45 |                 index.erase(unique(index.begin(),index.end()),index.end());
      |                       ^~~~~
a.cc:45:42: error: overloaded function with no contextual type information
   45 |                 index.erase(unique(index.begin(),index.end()),index.end());
      |                                          ^~~~~
a.cc:45:56: error: overloaded function with no contextual type information
   45 |                 index.erase(unique(index.begin(),index.end()),index.end());
      |                                                        ^~~
a.cc:45:69: error: overloaded function with no contextual type information
   45 |                 index.erase(unique(index.begin(),index.end()),index.end());
      |                                                                     ^~~
a.cc:47:37: error: overloaded function with no contextual type information
   47 |                 for(int i=0;i<index.size();i++){
      |                                     ^~~~
a.cc:48:37: error: invalid types '<unresolved overloaded function type>[int]' for array subscript
   48 |                         if(cnt[index[i]]>=2 && index[i].size()>=2){
      |                                     ^
a.cc:48:53: error: invalid types '<unresolved overloaded function type>[int]' for array subscript
   48 |                         if(cnt[index[i]]>=2 && index[i].size()>=2){
      |                                                     ^
a.cc:49:52: error: invalid types '<unresolved overloaded function type>[int]' for array subscript
   49 |                                 if(res.size()<index[i].size() || (res.size()==index[i].size() && index[i]<res)){
      |                                                    ^
a.cc:49:84: error: invalid types '<unresolved overloaded function type>[int]' for array subscript
   49 |                                 if(res.size()<index[i].size() || (res.size()==index[i].size() && index[i]<res)){
      |                                                                                    ^
a.cc:49:103: error: invalid types '<unresolved overloaded function type>[int]' for array subscript
   49 |                                 if(res.size()<index[i].size() || (res.size()==index[i].size() && index[i]<res)){
      |                                                                                                       ^
a.cc:50:50: error: invalid types '<unresolved overloaded function type>[int]' for array subscript
   50 |                                         res=index[i];
      |                                                  ^