Compilare: main.cpp:2:41: warning: ISO C++ forbids declaration of 'main' with no type [-Wreturn-type] int t,k[1<<20],n,i,j;char s[1<<20];main(){std::ifstream f("prefix.in");std::ofstream o("prefix.out");f>>t;while(f>>s){for(i=1,j=0;s[i];++i)if(s[i]==s[j])k[i]=++j;else k[i]=j=(s[i]==s[0]);for(j=0;i&&!j;--i)if(k[i-1]&&i%(i-k[i-1])==0)j=i;o<<j<<'\n';}} ^