A prime number greater than 5 always surrounds 6 and must satisfy 6*i-1 or 6*i+1; but a number satisfying 6*i-1 or 6*i+1 is not necessarily a prime number.
When judging, the number of strides in each step is 6.
int judg(int a) { int tmp=(int) Math.sqrt(a); if(a==1) return 0; if(a==2 || a==3) return 1; if(a%6!=1 &&a%6!=5) return 0; for(int i=5;i<=tmp;i+=6) { if(a%i==0||a%(i+2)==0) return 0; } return 1; }