Competitive_Programming/ETF_totient.cpp at master · apsc/Competitive_Programming · GitHub

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#include<stdio.h>
int at[1000000]={0};
int cntr=2;
int main(){
    int t;
    scanf("%d",&t);
    int n;

    while(t--){
               scanf("%d",&n);
               if(n<3){printf("1\n");continue;}
               if(n<5){printf("2\n");continue;}
               if(at[n]!=0){printf("%d\n",at[n]);continue;}
               int a[n+1];
               for(int i=2;i<=n;i++)a[i]=0;


               for(int i=cntr;i<=n;i++){
                       at[i]=i;
               }

               for(int i=2;i<=n;){
                       if(a[i]>0){
                                  if(i%6==1)i+=4;
                                  else if(i%6==5)i+=2;
                                  else i++;
                                  continue;
                       }
                       for(int j=i;j<=n;j+=i){
                               a[j]=i;
                               at[j]=(at[j]*(i-1))/i;
                       }
                       if(i%6==1)i+=4;
                       else if(i%6==5)i+=2;
                       else i++;
               }
               if(at[n]==n)
               printf("%d\n",at[n]-1);
               else
               printf("%d\n",at[n]);
               if(n>cntr)n=cntr;
    }

    return 0;
}