```
class Solution(object):
def findMissingRanges(self, nums, lower, upper):
res = []
nums = [lower-1] + nums + [upper+1]
left_p = 0
right_p = 1
while right_p < len(nums):
left = nums[left_p]
right = nums[right_p]
if left+1 == right-1:
res.append(str(left+1))
elif (right-1) - (left+1) > 0:
res.append(str(left+1) + "->" + str(right-1))
left_p += 1
right_p += 1
return res
```

Insert lower-1 and upper+1 into the two ends of num. And just compare two numbers at a time.