Interval [a,b] is covered by [c,d] if c <= a and b <= d. Given a list of intervals, remove all covered intervals and return the count of remaining intervals.
Example: [[1,4],[3,6],[2,8]]. The interval [1,4] is covered by [2,8]? No, because . But [[1,4],[2,8]] would have [1,4] NOT covered. Tricky: sorting strategy determines which intervals you can quickly identify as covered.