Problem:
Problem B: Prime Gap

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

Output for the Sample Input

4
0
6
0
114

Solution in C (judge verdict: Accepted)

#include <stdio.h>

#define IN_MAX 1299709

char table[IN_MAX+1];

void createtable1(){ //0->prime 1->not prime
  int i=2,j;
  for(;i<=IN_MAX;i++)
    if(!table[i])
      for(j=i*2;j<=IN_MAX;j+=i)
        table[j]=1;
}

void createtable2(){
  int i=2,j=2,x;
  for(;i<=IN_MAX;i++)
    if(!table[i]){
      for(x=j+1;x<i;x++)
        table[x]=i-j;
      j=i;
    }
}

int main(){
  int i;
  createtable1();
  createtable2();
  while(scanf("%d",&i)&&i)printf("%d\n",table[i]);
  return 0;
}