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

10
11
27
2
492170
0

Sample Output

4
0
6
0
114

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=4; j<maxn; j+=2)
        hash[j]=1;
    for(i=3; i<1200; i+=2)
        if(!hash[i])
            for(j=i*i; j<maxn; j+=i)
                hash[j]=1;
}
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(1)
        {
           if(isprime(i))
           break;
           i++;
        }
        int j=n;
        while(1)
        {
           if(isprime(j))
           break;
           j--;

        }
       printf("%d
",i-j);
    }
    return 0;
}