poj 3518 Prime Gap

Prime Gap

Time Limit: 5000MS		Memory Limit: 65536K
Total Submissions: 6120		Accepted: 3480

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

#include<iostream>
#include<cmath>
using namespace std;
int isPrime(int n)
{
    int i;
    if(n==1)
        return 1;
    if(n%2==0&&n!=2)
        return 0;
    for(i=2;i<=sqrt(n*1.0);i++)
    {
        if(n%i==0)
            return 0;
    }
    return 1;
}
int main()
{
    int n;
    int k;
    int max,min;
    int gap;
    while(1)
    {
        cin>>n;
        if(n==0)
            break;
        if(isPrime(n)==1)
            cout<<"0"<<endl;
        else
        {
            k=n+1;
            while(isPrime(k)==0)
            {
                k++;
            }
            max=k;
            k=n-1;
            while(isPrime(k)==0)
            {
                k--;
            }
            min=k;
            gap=max-min;
            cout<<gap<<endl;
        }
    }
    return 0;
}