[暑假集训--数论]poj3518 Prime Gap

The sequence of n − 1 consecutive composite numbers (positive integers that are not prime and not equal to 1) lying between two successive prime numbers p and p + n is called a prime gap of length n. For example, ‹24, 25, 26, 27, 28› between 23 and 29 is a prime gap of length 6.

Your mission is to write a program to calculate, for a given positive integer k, the length of the prime gap that contains k. For convenience, the length is considered 0 in case no prime gap contains k.

Input

The input is a sequence of lines each of which contains a single positive integer. Each positive integer is greater than 1 and less than or equal to the 100000th prime number, which is 1299709. The end of the input is indicated by a line containing a single zero.

Output

The output should be composed of lines each of which contains a single non-negative integer. It is the length of the prime gap that contains the corresponding positive integer in the input if it is a composite number, or 0 otherwise. No other characters should occur in the output.

Sample Input

10
11
27
2
492170
0

Sample Output

4
0
6
0
114

给个x，如果x夹在两个质数a,b之间，求b-a，否则输出0

在筛法的时候预处理下距离就好

#include<cstdio>
#include<iostream>
#include<cstring>
#define LL long long
using namespace std;
inline LL read()
{
    LL x=0,f=1;char ch=getchar();
    while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}
    while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();}
    return x*f;
}
LL n;
bool mk[2000010];
int p[2000010],len;
int ans[2000010];
inline LL LLabs(LL a){return a<0?-a:a;}
inline void getp()
{
    memset(ans,-1,sizeof(ans));
    for (int i=2;i<=2000000;i++)
    {
        if (!mk[i])
        {
            p[++len]=i;
            ans[i]=0;
            for (int j=2*i;j<=2000000;j+=i)mk[j]=1;
        }else ans[i]=ans[i-1]+1;
    }
    for (int i=2000000;i>=1;i--)
    {
        if (!ans[i])continue;
        ans[i]=max(ans[i],ans[i+1]);
    }
}
int main()
{
    getp();
    ans[1]=-1;
    while (~scanf("%lld",&n)&&n)printf("%d\n",ans[n]?ans[n]+1:0);
}