次大公约数

辗转相除法找最大公约数,然后用欧拉筛求出以内的质数与是否除尽来找答案

另好像有其他好方法，数论渣渣😪

#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define LL long long
#define IN freopen("in.txt", "r", stdin)
#define OUT freopen("out.txt", "w", stdout)
#define scan(x) scanf("%d", &x)
#define f(i, a, b) for (int i = a; i <= b; i++)
#define pn(x) printf("%d", x)
#define lowbit(x) (x & (-x))

int cnt = 0;
void Euler_prime(int n, bool *vis, int *prime)
{
    vis[1] = true;
    for (int i = 2; i <= n; ++i)
    {
        if (!vis[i])
            prime[cnt++] = i;
        for (int j = 0; j < cnt; ++j)
        {
            if (i * prime[j] > n) //判断是否越界
                break;
            vis[i * prime[j]] = true; //筛数
            if (i % prime[j] == 0)
                break;
        }
    }
}

int main()
{
    int n, x, min, max;
    scan(n);
    scan(x);
    min = n < x ? n : x;
    max = n > x ? n : x;
    while (max % min)
    {
        int temp = max % min;
        max = min;
        min = temp;
    }
    if (min == 1)
    {
        printf("-1");
        return 0;
    }
    int *prime = (int *)malloc(sizeof(int) * (min / 2));
    bool *vis = (bool *)malloc(sizeof(bool) * (min + 1));
    memset(vis, 0, sizeof(bool) * (min + 1));
    Euler_prime(min, vis, prime);
    f(i, 0, cnt - 1)
    {
        if ((min / prime[i]) * prime[i] == min)
        {
            pn(min / prime[i]);
            return 0;
        }
    }
    printf("1");
    return 0;
}