| Time Limit: 5000MS | Memory Limit: 65536K | |
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<stdio.h>
#include<math.h>
#include<iostream>
using namespace std;
int judge(int a)
{
if(a==0||a==1)
{
return 0;
}
int i;
for(i=2;i<=sqrt(a+1);i++)
{
if(a%i==0)
{
return 0;
}
}
return 1;
}
int main()
{
int a,b,c;
while(scanf("%d",&a),a)
{
if(judge(a)==1)
{
printf("0\n");
}
else
{
int i,j;
for(i=0;judge(a+i)!=1;i++)
for(j=0;judge(a-j)!=1;j++)
b=a+i+1;
c=a-j;
printf("%d\n",b-c);
}
}
return 0;
}