题目链接：
http://acm.zju.edu.cn/onlinejudge/showProblem.do?problemCode=2723
题目描述：
Prime Number Definition
An integer greater than one is called a prime number if its only
positive divisors (factors) are one and itself. For instance, 2, 11, 67,
89 are prime numbers but 8, 20, 27 are not.
Semi-Prime Number Definition
An integer greater than one is called a semi-prime number if it can be
decompounded to TWO prime numbers. For example, 6 is a semi-prime number
but 12 is not.
Your task is just to determinate whether a given number is a semi-prime number.
Input
There are several test cases in the input. Each case contains a single integer N (2 <= N <= 1,000,000)
Output
One line with a single integer for each case. If the number is a semi-prime number, then output "Yes", otherwise "No".
Sample Input
3
4
6
12
Sample Output
No
Yes
Yes
No
1 /*问题 判断输入的n（n的范围是2到100 0000）是否为半素数
2 解题思路 先用素筛法建立一张[2,50 0000]的素数表放在数组里，再将素数存进向量里，然后计算出2到100 0000的半素
3 数表放在set集合里后查表即可*/
4 #include <cstdio>
5 #include <set>
6 #include <cmath>
7 #include <vector>
8 using namespace std;
9
10 int isPrim[500010];
11
12 void prim(int n);
13
14 int main()
15 {
16 prim(500000);
17
18 vector<int> prim_vec;//对于未知大小的数据来说非常方便
19 int i;
20 for(i=2;i<=500000;i++){
21 if(isPrim[i])
22 prim_vec.push_back(i);
23 }
24
25 set<int> prim_set;//查询起来非常快速、方便
26 int j,p;
27 for(i=0;i<prim_vec.size();i++){
28 for(j=0;j<prim_vec.size();j++){
29 if( (p = prim_vec[i]*prim_vec[j]) <=1000000)
30 prim_set.insert(p);
31 else
32 break;//一种优化方法，对于有顺序的数据
33 }
34 }
35
36 int n;
37 set<int>::iterator it;
38 while(scanf("%d",&n) != EOF)
39 {
40 it=prim_set.find(n);
41 if(it != prim_set.end())
42 printf("Yes\n");
43 else
44 printf("No\n");
45 }
46 return 0;
47 }
48
49 void prim(int n)
50 {
51 int i,j,k;
52
53 for(i=0;i<=n;i++)
54 isPrim[i]=1;//全部初始化为1，假定全是素数
55 isPrim[0]=isPrim[1]=0;//0和1非素数
56
57 k=(int)sqrt(n);
58 for(i=2;i<=k;i++){
59 if(isPrim[i]){
60 for(j=2*i;j<=n;j+=i)
61 isPrim[j]=0;
62 }
63 }
64 /*
65 int count=0;
66 for(i=2;i<=n;i++){
67 if(isPrim[i])
68 count++;//记录50 0000以内的素数有41538个，验证素筛写的对不对
69 }
70 printf("%d\n",count);
71 */
72 }
原文地址：https://www.cnblogs.com/wenzhixin/p/8543728.html