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.

题意：求一个数是否是semi-prime number（其素数因子的个数恰好为2）

思路方法：素数筛选~边筛选边记录

#include<iostream>
#include<cstring>
#include<cmath>
#include<algorithm>
#include<vector>
#include<queue>
#include<stack>
#include<cstdlib>
#include<cstdio>
#include<set>
#include<map>
#define N 1000010
typedef long long LL;
using namespace std;
int p[N];
void prime(){
    memset(p,0,sizeof(p));
    p[0]=p[1]=0;
    for(int i=2;i<N;i++){
        if(!p[i]){
            p[i]=1;
            for(int j=i+i;j<N;j+=i){
                int k=j;
                while(k%i==0){
                    p[j]++;
                    k/=i;
                }
            }
        }
    }
}
int main(){
    prime();
    int n;
    while(scanf("%d",&n)!=EOF){
        printf("%s\n",p[n]==2?"Yes":"No");
    }
    return 0;
}