hdu 5104 Primes Problem (素数+递推）

Problem Description

Given a number n, please count how many tuple(p1, p2, p3) satisfied that p1<=p2<=p3, p1,p2,p3 are primes and p1 + p2 + p3 = n.

 

Input

Multiple test cases(less than 100), for each test case, the only line indicates the positive integer n(n≤10000).

 

Output

For each test case, print the number of ways.

 

Sample Input

3

9

 

Sample Output

0

2

   先找出所有素数，再递推。

#include<cstdio>
#include<cstring>
using namespace std;
const int N=10005;
int zj[N],ssb[N],pn,ans,n;
void f()
{
    int i,j;
    zj[0]=zj[1]=1;
    pn=0;
    for (i=2;i<N;i++)
    {
        if (!zj[i]) ssb[pn++]=i;
        for (j=0;j<pn;j++)
        {
            if (i*ssb[j]>N) break;
            zj[i*ssb[j]]=1;
            if (i%ssb[j]==0) break;
        }
    }
}
void dfs(int sum,int x,int p)
{
    if (p==2)
    {
        if (zj[n-sum]==0&&n-sum>=ssb[x]) ans++;
        return ;
    }
    int i;
    for (i=x;i<pn;i++)
    {
        if (sum+ssb[i]>=n) return ;
        dfs(sum+ssb[i],i,p+1);
    }
}
int main()
{
    f();
    while (~scanf("%d",&n))
    {
        ans=0;
        dfs(0,0,0);
        printf("%d
",ans);
    }
}