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打表只需79ms

Posted by lcystar at 2011-09-01 20:38:45 on Problem 3518
#include<stdio.h>
#include<math.h>
#define M 1299710
int a[M],p[M];
int main()
{
	int q,i,j,n,t=0;
	q=(int)sqrt(M*1.0);
	for(i=2;i<M;i++)a[i]=1;
	for(i=2;i<=q;i++)
		if(a[i])for(j=i+i;j<M;j+=i)a[j]=0;
	for(i=2;i<M;i++){
		if(a[i]){
			a[i]=0;
			p[++t]=i;
		}
		else a[i]=t;
	}
	while(scanf("%d",&n),n){
		if(!a[n])puts("0");
		else printf("%d\n",p[a[n]+1]-p[a[n]]);
	}
	return 0;
}

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