题目链接：https://www.zhixincode.com/problem/day2-2

       数据范围太大，需要用unsigned long long，不知道为什么字符串的方法过不了..

AC代码：

#include <bits/stdc++.h>
#define ll unsigned long long
#define maxn 6005
using namespace std;
ll pre[maxn];
int T,n;

ll fun(ll a, ll b){
  ll xx = b, cnt = 1;
  while(xx){
    xx /= 10;
    cnt *= 10;
  }
  a *= cnt;
  return a + b;
}

int main()
{
  int Case = 1;
  scanf("%d",&T);
  while(T--){
    scanf("%d",&n);
    for(int i=0;i<n;i++){
      scanf("%llu",&pre[i]);
    }
    ll ans = 0;
    for(int i=0;i<n;i++){
      for(int j=i+1;j<n;j++){
        ans = max(ans, fun(pre[i], pre[j]));
      }
    }
    printf("Case #%d: %llu\n",Case++, ans);
  }
  return 0;
}