hihocoder offer收割编程练习赛12 A 歌德巴赫猜想

思路：

枚举。

实现：

#include <iostream>
#include <cstdio>
#include <algorithm>
using namespace std;

const int MAX_N = 1000005;

int prime[MAX_N];
bool is_prime[MAX_N + 1];

int init(int n)
{
    int p = 0;
    for (int i = 0; i <= n; i++)
        is_prime[i] = true;
    is_prime[0] = is_prime[1] = false;
    for (int i = 2; i <= n; i++)
    {
        if (is_prime[i])
        {
            prime[p++] = i;
            for (int j = 2 * i; j <= n; j += i)
                is_prime[j] = false;
        }
    }
    return p;
}

int main()
{
    int num = init(1000000);
    int n;
    cin >> n;
    for (int i = 0; i < num; i++)
    {
        if (binary_search(prime + i, prime + num, n - prime[i]))
        {
            int index = lower_bound(prime + i, prime + num, n - prime[i]) - prime;
            cout << prime[i] << " " << prime[index] << endl;
            break;
        }
    }
    return 0;
}