Given a sorted integer array where the range of elements are in the inclusive range [lower, upper], return its missing ranges.
For example, given [0, 1, 3, 50, 75], lower = 0 and upper = 99, return ["2", "4->49", "51->74", "76->99"].
[0, 1, 3, 50, 75]
["2", "4->49", "51->74", "76->99"].