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
== 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 ;

}

}