poj3518

线性筛法可以过

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
using namespace std;


const int  MAXSIZE=1500000;
unsigned long previous[MAXSIZE+1];   /*size of array is MAXSIZE, then previous*/
unsigned long next[MAXSIZE+1];         /*and next[MAXSIZE] can be used for */
unsigned long prime, fact, i, mult;         /*number MAXSIZE*/
unsigned long n;
bool notprime[MAXSIZE+1];
inline void REMOVE(unsigned long x)
{
       next[previous[x]]=next[x];
       previous[next[x]]=previous[x];
       notprime[x] = true;
}
inline void INITIAL(unsigned long n)
{
   unsigned long i=3;
   memset(notprime, 0, sizeof(notprime));
   notprime[0] = true;
   notprime[1] = true;
   while(i<=n)
   {
     previous[i]=i-2;
     if(i+2<=n)
     {
         next[i]=i+2;
         notprime[i + 1] = true;
         i+=2;
     }
     else
     {
          next[i]=0;
          break;
     }
   }
   previous[3]=2;
   next[2]=3;
   previous[2]=0;
}
int main(void)
{
	n = 1500000;
	INITIAL(n);
    for(prime=3;prime*prime<=n;prime=next[prime])
        for(fact=prime;prime*fact<=n;fact=next[fact])
            for(mult=prime*fact;mult<=n;mult*=prime)
                   REMOVE(mult);
    int k;
    while (scanf("%d", &k) != EOF && k != 0)
    {
    	int l = k, r = k;
    	if (!notprime[k])
    	{
    		printf("0\n");
    		continue;
    	}
    	while (notprime[l])
    		l--;
    	while (notprime[r])
    	    r++;
    	printf("%d\n", r - l);
    }
    return 0;
}