T = int(input())
for _ in range(T):
    N = int(input())
    A = list(map(int,input().split()))
    high = 10**10
    low = -1
    while high-low>1:
        mid = (high+low)//2
        flag = True
        for i in range(1,N):
            if A[i-1]+i*mid<A[i]+(i+1)*mid:continue
            flag = False
            break
        if flag:
            high = mid
        else:
            low = mid
    print(high)