getting a different number

 1def get_different_number(arr):
 2  if len(arr) == 1:
 3    if arr[0] > 0:
 4      return 0
 5    else:
 6      return 1
 7  mi = min(arr)
 8  ma = max(arr)
 9  if mi != 0:
10    return 0
11  s = set(arr)
12  for i in range(mi, ma+1):
13    if i not in s:
14      return i
15  return ma + 1