from bisect import bisect_left

def solve():
    n = int(input())
    a = list(map(int, input().split(' ')))
    # print(n,a)
    ans = 0
    for i in range(n-1):
        ng = -1
        ok = int(1e18)
        while abs(ng-ok) > 1:
            mid = ng+ok
            mid //= 2
            if a[i] + (i+1)*mid < a[i+1] + (i+2)*mid:
                ok = mid
            else:
                ng = mid
        # print('ok',ok)
        ans = max(ans,ok)
    # print(ans)
    return ans

t = int(input())
for _ in range(t):
    print(solve())