https://www.luogu.com.cn/record/126392114

#include <iostream>
#include <algorithm>
#define big long long
using namespace std;
big n,a[2000007],cnt=0,b[2000007];
int main()
{
	cin >> n;
	for(big i = 1;i <= n;i++)
	{
		cin >> a[i];
	}
    sort(a+1,a+n+1);
    n = unique(a+1,a+n+1)-a-1;
	for(big i = 1;i <= n;i++)
	{
		for(big j = 1;j < n;j++)
		{
			b[++cnt] = a[i]%a[j];
            b[++cnt] = a[j]%a[i];
		}
	}
    sort(b+1,b+cnt+1);
    big len = unique(b+1,b+cnt+1)-b-1;
    cout << b[len-1];
	return 0;
}