hdu 3823 线性筛法

筛出足够多的素数然后存起来查找即可。

 1 #include <algorithm>
 2 #include <iostream>
 3 #include <cstring>
 4 #include <cstdio>
 5 using namespace std;
 6
 7 typedef long long ll;
 8 const int N = 20000001;
 9 const int M = 1500000;
10 const int K = 150;
11 const int L = 200000;
12 bool visit[N];
13 int prime[M];
14 int ss[K][L];
15 int pm[K];
16 int pn;
17
18 void better_get_prime()
19 {
20     pn = 0;
21     memset( visit, 0, sizeof(visit) );
22     visit[0] = visit[1] = 1;
23     for ( int i = 2; i < N; i++ )
24     {
25         if ( !visit[i] ) prime[pn++] = i;
26         for ( int j = 0; j < pn && ( ll ) i * prime[j] < N; j++ )
27         {
28             visit[i * prime[j]] = 1;
29             if ( i % prime[j] == 0 ) break;
30         }
31     }
32     memset( pm, 0, sizeof(pm) );
33     for ( int i = 0; i < pn - 1; i++ )
34     {
35         int d = prime[i + 1] - prime[i];
36         if ( d < K )
37         {
38             ss[d][pm[d]++] = prime[i];
39         }
40     }
41 }
42
43 int main ()
44 {
45     better_get_prime();
46     int t;
47     scanf("%d", &t);
48     for ( int _case = 1; _case <= t; _case++ )
49     {
50         int x, y;
51         scanf("%d%d", &x, &y);
52         if ( x > y ) swap( x, y );
53         int d = y - x, ans = -1;
54         for ( int i = 0; i < pm[d]; i++ )
55         {
56             if ( ss[d][i] >= x )
57             {
58                 ans = ss[d][i] - x;
59                 break;
60             }
61         }
62         printf("Case %d: %d\n", _case, ans);
63
64     }
65     return 0;
66 }