friends.cpp: In function 'bool c()':
friends.cpp:5:36: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
5 | bool c(){p=0;for(i=l;i<=r;i++){if(p!=t.size()&&t[p]==s[i])p++;}return(p==t.size());}int main(){cin>>n>>s;if(!(n&1)){cout << "NOT POSSIBLE";return 0;}t=s.substr(0,n/2);l=n/2;r=n-1;f=c();t=s.substr(n-n/2,n/2);l=0;r=n/2;g=c();if(!f&&!g)cout<<"NOT POSSIBLE";else if(f&&g&&s.substr(0,n/2)!=s.substr(n-n/2,n/2))cout<<"NOT UNIQUE";else if(f)cout<<s.substr(0,n/2);else cout<<s.substr(n-n/2,n/2);}
| ~^~~~~~~~~~
friends.cpp:5:72: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
5 | bool c(){p=0;for(i=l;i<=r;i++){if(p!=t.size()&&t[p]==s[i])p++;}return(p==t.size());}int main(){cin>>n>>s;if(!(n&1)){cout << "NOT POSSIBLE";return 0;}t=s.substr(0,n/2);l=n/2;r=n-1;f=c();t=s.substr(n-n/2,n/2);l=0;r=n/2;g=c();if(!f&&!g)cout<<"NOT POSSIBLE";else if(f&&g&&s.substr(0,n/2)!=s.substr(n-n/2,n/2))cout<<"NOT UNIQUE";else if(f)cout<<s.substr(0,n/2);else cout<<s.substr(n-n/2,n/2);}
| ~^~~~~~~~~~