t=int(input())
for _ in range(t):
  n=int(input())
  a=list(map(int,input().split()))
  ok=10**9
  ng=-1
  while ok-ng>1:
    m=(ok+ng)//2
    b=[a[i]+(i+1)*m for i in range(n)]
    if all(b[i]<b[i+1] for i in range(n-1)):
      ok=m
    else:
      ng=m
  print(ok)