hdu 3823 线性筛法

筛出足够多的素数然后存起来查找即可。

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

typedef long long ll;
const int N = 20000001;
const int M = 1500000;
const int K = 150;
const int L = 200000;
bool visit[N];
int prime[M];
int ss[K][L];
int pm[K];
int pn;

void better_get_prime()
{
    pn = 0;
    memset( visit, 0, sizeof(visit) );
    visit[0] = visit[1] = 1;
    for ( int i = 2; i < N; i++ )
    {
        if ( !visit[i] ) prime[pn++] = i;
        for ( int j = 0; j < pn && ( ll ) i * prime[j] < N; j++ )
        {
            visit[i * prime[j]] = 1;
            if ( i % prime[j] == 0 ) break;
        }
    }
    memset( pm, 0, sizeof(pm) );
    for ( int i = 0; i < pn - 1; i++ )
    {
        int d = prime[i + 1] - prime[i];
        if ( d < K )
        {
            ss[d][pm[d]++] = prime[i];
        }
    }
}

int main ()
{
    better_get_prime();
    int t;
    scanf("%d", &t);
    for ( int _case = 1; _case <= t; _case++ )
    {
        int x, y;
        scanf("%d%d", &x, &y);
        if ( x > y ) swap( x, y );
        int d = y - x, ans = -1;
        for ( int i = 0; i < pm[d]; i++ )
        {
            if ( ss[d][i] >= x )
            {
                ans = ss[d][i] - x;
                break;
            }
        }
        printf("Case %d: %d
", _case, ans);

    }
    return 0;
}