def nibun(f):
    left,right=-1,10**9
    while right-left>1:
        mid=(right+left)//2
        if f(mid):
            left=mid
        else:
            right=mid
    return right

def isok(n):
    B=[A[i]+i*n for i in range(N)]
    for i in range(N-1):
        if B[i]>=B[i+1]:
            return True
    return False

T=int(input())
for _ in range(T):
    N=int(input())
    A=list(map(int,input().split()))
    print(nibun(isok))