Python找出列表中不连续的数

比如：

a = [1, 2, 4]输出3

a = [1, 2, 3, 4, 8, 9, 12] 只输出5

求解：

>>> a = [1, 2, 3, 4, 8, 9, 12]
>>> set(range(a[0],a[-1]+1))-set(a)
{10, 11, 5, 6, 7}
>>> sorted(list(set(range(a[0],a[-1]+1))-set(a)))
[5, 6, 7, 10, 11]
>>> sorted(list(set(range(a[0], a[-1]+1)) - set(a)), reverse=True)
[11, 10, 7, 6, 5]
>>> sorted(list(set(range(a[0], a[-1]+1)) - set(a)), reverse=True).pop()
5