lightoj - 1189 - Sum of Factorials

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
int main()
{
    unsigned long long fact[22];
    fact[0]=1;
    for(int i=1;i<=20;i++)fact[i]=fact[i-1]*i;

    ll test;
    cin>>test;
    for(ll c=1;c<=test;c++)
    {
        ll x,i;
        cin>>x;
        stack<ll>s;
        i=20;
        while(i>=0)
        {
            if(fact[i]<=x)
            {
                s.push(i);
                x-=fact[i];
            }
            i--;
        }
        if(x==0)
        {
            printf("Case %lld: ",c);
            while(s.size()!=1)
            {
                x=s.top();
                printf("%d!+",x);
                s.pop();
            }
            x=s.top();
            s.pop();
            printf("%lld!\n",x);
        }
        else
        {
            printf("Case %lld: impossible\n",c);
        }
    }
}