Given a sorted integer arraynums, where the range of elements are in theinclusive range[lower,upper], return its missing ranges.

Example:

Input:
nums
 = 
[0, 1, 3, 50, 75]
, 
lower
 = 0 and 
upper
 = 99,

Output:
["2", "4-
>
49", "51-
>
74", "76-
>
99"]

class Solution(object):
    def findMissingRanges(self, A, lower, upper):
        result = []
        A.append(upper+1)
        pre = lower - 1
        for i in A:
           if (i == pre + 2):
               result.append(str(i-1))
           elif (i > pre + 2):
               result.append(str(pre + 1) + "->" + str(i -1))
           pre = i
        return result