找各元素之间的最大差值

For a array of integer X_i, 0 <= i <= N, solve for the value of max(X_i - X_j) for all i > j
for example: [1,2,3,4,5] -> 4; [5,4,3,2,1] -> -1
（Ideal time complexity is O(N) ）

def max_val(array):
    num = -1000000
    for i in range(len(array)-1):
        a = array[i+1:]
        a.sort(reverse=True)
        maxval = a[0] - array[i]
        num = max(num,maxval)
    return num
 
print(max_val([8,2,6,4,5]))