var
  t,w,n,i,x:longint;
  a,b:array[1..500000] of longint;
procedure qsort(l,r:longint);
var
  i,j,m,t:longint;
begin
  i:=l;j:=r;m:=a[(l+r)div 2];
  repeat
    while a[i]<m do i:=i+1;
    while a[j]>m do j:=j-1;
    if i<=j then
    begin
      t:=a[i];a[i]:=a[j];a[j]:=t;
      inc(i);dec(j);
    end;
  until i>j;
  if l<j then qsort(l,j);
  if i<r then qsort(i,r);
end;
begin
  readln(t);
  for w:=1 to t do
  begin
    readln(n);
    for i:=1 to n do read(a[i]);
    qsort(1,n);
    x:=a[1];
    for i:=1 to n do
    begin
      b[i]:=x;
      if x<a[i+1]-b[i] then x:=a[i+1]-b[i];
    end;
    writeln(b[n]);
  end;
end.