素数筛 poj 3518

#include<stdio.h>
#include<string.h>
#include<algorithm>
#include<vector>
#include<math.h>

using namespace std;
#define MAXN 1302000
bool pri[MAXN];

int main()
{
    memset(pri,0,sizeof(pri));

    for(int i=2;i<=1141;i++)
    {
        if(!pri[i])
            for(int j=i*i;j<=MAXN;j=j+i)
                pri[j]=1;
    }
    int n;

    while(scanf("%d",&n)!=EOF&&n)
    {
        int st=n,en=n;
        while(pri[st]==1)
            st--;
        while(pri[en]==1)
            en++;
        printf("%d\n",en-st);
    }

    return 0;
}