```
class Solution(object):
def findMissingRanges(self, nums, lower, upper):
nums.insert(0,lower-1)
nums.append(upper+1)
i,j = 0,1
res = []
while j < len(nums):
if nums[j] - nums[i] == 2:
res.append(str(nums[i]+1))
elif nums[j] - nums[i] > 2:
res.append(str(nums[i]+1) + '->' +str(nums[j]-1))
i += 1
j += 1
return res
```