Semi-Prime(半素数）

http://acm.zju.edu.cn/onlinejudge/showProblem.do?problemCode=2723

Semi-Prime

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Time Limit: 2 Seconds      Memory Limit: 65536 KB

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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

思路：如果一个数能分解为两个素数的乘积（大于1），那么这个数就是半素数。建立一个【2，500000】的素数集合，在建立一个【1，1000000】的半素数集合，

set是平衡检索二叉树，检索速度足够。

#include <iostream>
#include <cstdio>
#include <cstring>
#include <queue>
#include <cmath>
#include <vector>
#include <set>
#include <map>
#include <algorithm>
using namespace std;
typedef long long ll;
vector<int>v;
set<int>s;
void get_prime(int b)
{
    int a[500009];
    memset(a,0,sizeof(a));
    a[1]=1;
    for(int i=2;i<=b;i++)
    {
        if(a[i]==1) continue;
        v.push_back(i);
        for(int j=2;j*i<=b;j++)
        {
            a[i*j]=1;
        }
    }
}
void get_no_prime(int a)
{
    for(int i=0;i<v.size();i++)
    {
        for(int j=0;j<v.size();j++)
        {
            int ans=v[i]*v[j];
            if(ans<a) s.insert(ans);
            else break;
        }
    }  
}
int main()
{
    int n;
    get_prime(500000);
    get_no_prime(1000000);
    while(scanf("%d",&n)!=EOF)
    {
        puts(s.find(n)!=s.end()?"Yes":"No");
    }
    return 0;
}