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 }
