poj 3518 Prime Gap

Prime Gap

Time Limit: 5000MS		Memory Limit: 65536K
Total Submissions: 7392		Accepted: 4291

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.

Sample Input

Sample Output

Source

Japan 2007

#include <iostream>

#include<stdio.h>

#include<stdlib.h>

using namespace std;

#define maxn 1300000

bool hash[maxn];

void inithash()

{

    int i,j;

    for(j=; j<maxn; j+=)

        hash[j]=;

    for(i=; i<; i+=)

        if(!hash[i])

            for(j=i*i; j<maxn; j+=i)

                hash[j]=;

}

int isprime(int N)

{

        if(!hash[N])

            return true;

    return false;

}

int main()

{

    inithash();

    int n;

    while(scanf("%d",&n),n)

    {

        int i=n;

        while()

        {

           if(isprime(i))

           break;

           i++;

        }

        int j=n;

        while()

        {

           if(isprime(j))

           break;

           j--;

        }

       printf("%d\n",i-j);

    }

    return ;

}

巴特西

poj 3518 Prime Gap

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