#include <iostream>
#include <algorithm>
#include <cstdio>
#include <cstring>
using namespace std;
#define int long long
const int maxn=2e3+10;
int a[maxn];
int b[maxn];
void solve(){
    int n;
    scanf("%lld",&n);
    a[0]=0;
    for(int i=1;i<=n;i++){
        scanf("%lld",&a[i]);
    }
    if(n==1){cout<<a[1]<<endl;return;}
    sort(a+1,a+1+n);
    int ans=0;
    for(int i=1;i<=n;i++){
        b[i]=a[i]-a[i-1];
        ans=max(b[i],ans);
    }
    cout<<ans<<endl;
}
signed main(){
    int T;
    scanf("%lld",&T);
    while(T--)solve();
    return 0;
}