求第N个素数

Posted 2020-07-14 Ashly

题意:

　　输入一个数字N(N <= 200000),输出底N个素数.

思路:

　　1-3000000中大约有210000素数.直接把这些素数打表,然后离线查询就OK.

代码:

 1 #include <iostream>
 2 #include <cmath>
 3 #include <cstdio>
 4 #include <cstring>
 5 #include <cstdlib>
 6 #include <algorithm>
 7 #include <queue>
 8 #include <stack>
 9 #include <vector>
10 using namespace std;
11 typedef long long LL;
12 
13 bool visit[3010000 + 7];  
14 int prime[3000000 + 7];  
15 int ans[220000 + 7];
16 void init_prim(){  
17     memset(visit, true, sizeof(visit));  
18     int num = 0;  
19     for (int i = 2; i <= 3000000; ++i){  
20         if (visit[i] == true){  
21             num++;  
22             prime[num] = i;  
23         }  
24         for (int j = 1; ((j <= num) && (i * prime[j] <= 3000000));  ++j){  
25             visit[i * prime[j]] = false;  
26             if (i % prime[j] == 0) break; 
27         }  
28     }  
29 }
30 
31 int main()
32 {
33     //freopen("input.txt", "r", stdin);
34     memset(prime, 0, sizeof(prime));
35     init_prim();
36     int cnt = 0;
37     for(int i = 0; i <= 3000000; i++){
38         if(prime[i])ans[cnt++] = prime[i];
39     }
40     int t;
41     scanf("%d", &t);
42     while(t--){
43         int n;
44         scanf("%d", &n);
45         printf("%d\n", ans[n - 1]);
46     }
47     return 0;
48 }