Simulate what it says: go from lower to upper.
class Solution:
def findMissingRanges(self, nums: List[int], lower: int, upper: int) -> List[List[int]]:
ans = []
for num in nums:
if num > lower:
ans.append([lower, num - 1])
lower = num + 1
if lower <= upper:
ans.append([lower, upper])
return ans