for _ in range(int(input())):
    N = int(input())
    A = [-1] + list(map(int,input().split()))
    OK = 10**18
    NG = -1
    while OK - NG >= 2:
        MID = (OK+NG)//2
        X = [A[i]+i*MID for i in range(1,N+1)]
        flag = True
        for i in range(N-1):
            if X[i] >= X[i+1]:
                flag = False
        if flag:
            OK = MID
        else:
            NG = MID
    print(OK)