Prime Gap

http://poj.org/problem?id=3518

// File Name: poj3518.cpp
// Author: bo_jwolf
// Created Time: 2013年10月11日 星期五 15:46:48

#include<vector>
#include<list>
#include<map>
#include<set>
#include<deque>
#include<stack>
#include<bitset>
#include<algorithm>
#include<functional>
#include<numeric>
#include<utility>
#include<sstream>
#include<iostream>
#include<iomanip>
#include<cstdio>
#include<cmath>
#include<cstdlib>
#include<cstring>
#include<ctime>

using namespace std;
const int maxn = 1399709;
vector< int > prime;
int unprime[ maxn ];

void Union(){
	memset( unprime, 0, sizeof( unprime ) );
	unprime[ 0 ] = unprime[ 1 ] = true;
	for( int i = 2; i < maxn; ++i ){
		if( !unprime[ i ] ){
			prime.push_back( i );
			for( int j = i + i; j < maxn; j += i ){
				unprime[ j ] = true;
			}
		}
	}
}

int main(){
	Union();
	int n, l, r, mid;
	while( cin >> n ){
		if( !n )
			break;
		int ans = 0;
		l = 0;
		r = prime.size() - 1;
		for( int i = 0; i < prime.size(); ++i ){
			if( prime[ i ] < n && prime[ i + 1 ] > n ){
				ans = prime[ i + 1 ] - prime[ i ];
			}
		}
		cout << ans << endl;
	}
return 0;
}