Language: C++

include<bits/stdc++.h>
using namespace std;
typedef pair<string,string> P;
string s[1001];
bool b[1001];
vector<P>v;
vector<string>a;
int n,m;
main(){
  cin>>n>>m;
  for(int i=0;i<n;i++)cin>>s[i];

  for(int i=0;i<n;i++)if(!b[i]){
    reverse(s[i].begin(),s[i].end());

    for(int j=i+1;j<n;j++)if(!b[j]&&s[j]==s[i]){
      reverse(s[j].begin(),s[j].end());
      v.push_back(P(s[i],s[j]));
      b[j]=b[i]=1;
      break;
    }
  }
  for(int i=0;i<n;i++)if(!b[i]){
    string t=s[i];
    reverse(s[i].begin(),s[i].end());
    if(s[i]==t)a.push_back(t);
  }

  for(int i=0;i<v.size();i++)
    if(v[i].second<v[i].first)swap(v[i].first,v[i].second);

  sort(a.begin(),a.end());
  sort(v.begin(),v.end());

  for(int i=0;i<v.size();i++)cout<<v[i].first;
  if(a.size())cout<<a[0];
  for(int i=v.size()-1;i>=0;i--)cout<<v[i].second;
  cout<<endl;
}

Compiler output:
a.cc:1:1: error: 'include' does not name a type
    1 | include<bits/stdc++.h>
      | ^~~~~~~
a.cc:3:9: error: 'pair' does not name a type
    3 | typedef pair<string,string> P;
      |         ^~~~
a.cc:4:1: error: 'string' does not name a type
    4 | string s[1001];
      | ^~~~~~
a.cc:6:1: error: 'vector' does not name a type
    6 | vector<P>v;
      | ^~~~~~
a.cc:7:1: error: 'vector' does not name a type
    7 | vector<string>a;
      | ^~~~~~
a.cc:9:1: warning: ISO C++ forbids declaration of 'main' with no type [-Wreturn-type]
    9 | main(){
      | ^~~~
a.cc: In function 'int main()':
a.cc:10:3: error: 'cin' was not declared in this scope
   10 |   cin>>n>>m;
      |   ^~~
a.cc:11:28: error: 's' was not declared in this scope
   11 |   for(int i=0;i<n;i++)cin>>s[i];
      |                            ^
a.cc:14:13: error: 's' was not declared in this scope
   14 |     reverse(s[i].begin(),s[i].end());
      |             ^
a.cc:14:5: error: 'reverse' was not declared in this scope
   14 |     reverse(s[i].begin(),s[i].end());
      |     ^~~~~~~
a.cc:18:7: error: 'v' was not declared in this scope
   18 |       v.push_back(P(s[i],s[j]));
      |       ^
a.cc:18:19: error: 'P' was not declared in this scope
   18 |       v.push_back(P(s[i],s[j]));
      |                   ^
a.cc:24:5: error: 'string' was not declared in this scope
   24 |     string t=s[i];
      |     ^~~~~~
a.cc:25:13: error: 's' was not declared in this scope
   25 |     reverse(s[i].begin(),s[i].end());
      |             ^
a.cc:25:5: error: 'reverse' was not declared in this scope
   25 |     reverse(s[i].begin(),s[i].end());
      |     ^~~~~~~
a.cc:26:14: error: 't' was not declared in this scope
   26 |     if(s[i]==t)a.push_back(t);
      |              ^
a.cc:26:16: error: 'a' was not declared in this scope
   26 |     if(s[i]==t)a.push_back(t);
      |                ^
a.cc:29:17: error: 'v' was not declared in this scope
   29 |   for(int i=0;i<v.size();i++)
      |                 ^
a.cc:30:31: error: 'swap' was not declared in this scope
   30 |     if(v[i].second<v[i].first)swap(v[i].first,v[i].second);
      |                               ^~~~
a.cc:32:8: error: 'a' was not declared in this scope
   32 |   sort(a.begin(),a.end());
      |        ^
a.cc:32:3: error: 'sort' was not declared in this scope; did you mean 'short'?
   32 |   sort(a.begin(),a.end());
      |   ^~~~
      |   short
a.cc:33:8: error: 'v' was not declared in this scope
   33 |   sort(v.begin(),v.end());
      |        ^
a.cc:35:30: error: 'cout' was not declared in this scope
   35 |   for(int i=0;i<v.size();i++)cout<<v[i].first;
      |                              ^~~~
a.cc:36:15: error: 'cout' was not declared in this scope
   36 |   if(a.size())cout<<a[0];
      |               ^~~~
a.cc:37:33: error: 'cout' was not declared in this scope
   37 |   for(int i=v.size()-1;i>=0;i--)cout<<v[i].second;
      |                                 ^~~~
a.cc:38:3: error: 'cout' was not declared in this scope
   38 |   cout<<endl;
      |   ^~~~
a.cc:38:9: error: 'endl' was not declared in this scope
   38 |   cout<<endl;
      |         ^~~~