PKU Prime Gap 3518 简单素数

/*
    stratege : 素数
    URL:http://poj.org/problem?id=3518
    
    Problem: 3518		User: a312745658
    Memory: 5684K		Time: 94MS
    Language: C++		Result: Accepted

*/

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdio>
using namespace std ;

int a[1299710] ;
int prime[100001] ;
int n ;

int main()
{
    int i, j, k, num = 0 ;
    int tmp ;
    int cnt = 0 ;
    int left, right ;

    for (i = 0; i < 1299710; i ++)
        a[i] = 0 ;

    for (i = 2; i < 1299710; i ++)
    {
        if (a[i] == 0)
        {
            tmp = 2 * i ;
            while (tmp < 1299710)
            {
                a[tmp] = 1 ;
                tmp += i ;
            }
            prime[num++] = i ;
        }
    }

    while (cin >> n && n)
    {
        if (a[n] == 0)
        {
            cout << 0 << endl ;
            continue ;
        }
        for (i = n-1; i >= 2; i --)
            if (a[i] == 0)
            {
                left = i ;
                break ;
            }
        for (i = n+1; i < 1299710; i ++)
            if (a[i] == 0)
            {
                right = i ;
                break ;
            }
        cout << right - left << endl ;

    }

}