学步园

题目链接：hdu 5104 Primes Problem

题目大意：给定n，判断有多少组素数p1+p2+p3=n（p1<=p2<=p3）

解题思路：n的范围给定，先筛选出素数，然后枚举前两个，判断一下。

#include <cstdio>
#include <cstring>
#include <algorithm>

using namespace std;

const int maxn = 10000;

int np, pri[maxn+5], vis[maxn+5];

void prime_table(int n) {

    np = 0;
    memset(vis, 0, sizeof(vis));
    vis[0] = vis[1] = 1;

    for (int i = 2; i <= n; i++) {
        if (vis[i])
            continue;

        pri[np++] = i;
        for (int j = i * i; j <= n; j += i)
            vis[j] = 1;
    }
}

int main () {
    int n;
    prime_table(maxn);
    while (scanf("%d", &n) == 1) {
        int ans = 0;
        for (int i = 0; i < np; i++) {
            for (int j = i; j < np; j++) {
                if (pri[i] + pri[j] >= n)
                    break;
                int t = n - pri[i] - pri[j];

                if (vis[t] == 0 && t >= pri[j])
                    ans++;
            }
        }
        printf("%d\n", ans);
    }
    return 0;
}