【luogu P1865 A % B Problem】 题解

题目链接：https://www.luogu.org/problemnew/show/P1865

其实就是埃拉托色尼筛素数模板...

好像每个数暴力枚举到sqrt()也可以...就算当我无聊练手罢

 #include <cstdio>
 #include <cmath>
 #include <algorithm>
 #include <cstring>
 using namespace std;
 const int maxn =  + ;
 bool prime[maxn];
 int n, m, left, right, sq, tot = ;
 int main()
 {
     memset(prime,,sizeof(prime));
     scanf("%d%d", &n, &m);
 
     sq = sqrt(m);
     prime[] = ;
     for(int i = ; i <= sq; i++)
         if(prime[i] == )
         {
             for(int j = i*i; j <= m; j+=i)
             prime[j] = ;
         }
 
     for(int i = ; i <= n; i++)
     {
         scanf("%d%d", &left, &right);
         if(left > m || right > m || left <=  || right <= )
         {
             printf("Crossing the line\n");
             continue;
         }
         else
         {
             for(int j = left; j <= right; j++)
             {
                 if(prime[j] == )
                 tot++;
             }
             printf("%d\n",tot);
             tot = ;
         }
 
     }
     return ;
 }