poj3518 Prime Gap

Description

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.

线性筛出所有质数，然后二分查找最接近的。

#include<cstdio>#include<cstring>const int maxn=1300000;bool have[maxn+10];int prm[101000],tot;int main(){    int i,j,k,m,n,p,q,x,y,z,l,r,mid;    for (i=2;i<=maxn;i++)    {        if (!have[i]) prm[++tot]=i;        for (j=1;j<=tot&&(long long)i*prm[j]<=maxn;j++)        {            have[i*prm[j]]=1;            if (i%prm[j]==0) break;        }    }    while (scanf("%d",&n)&&n)      if (!have[n]) printf("0\n");      else      {        l=1;        r=tot;        while (l<r)        {            mid=(l+r+1)/2;            if (prm[mid]>=n) r=mid-1;            else l=mid;        }        printf("%d\n",prm[l+1]-prm[l]);      }}