import sys
input = sys.stdin.readline

def check(m, B):
    M = len(B)
    temp = [0] * M
    for i in range(M):
        temp[i] = B[i] + i * m
    for i in range(M - 1):
        if temp[i] >= temp[i + 1]:
            return False
    return True 

T = int(input())
for _ in range(T):
    N = int(input())
    A = list(map(int, input().split()))
    yes = 10 ** 18
    no = -1
    while yes - no != 1:
        mid = (yes + no)//2
        if check(mid, A):
            yes = mid
        else:
            no = mid
    print(yes)