Given an array of meeting time intervals consisting of start and end times [[s1,e1],[s2,e2],...] (si < ei), find the minimum number of conference rooms required.
[[s1,e1],[s2,e2],...]
For example, Given [[0, 30],[5, 10],[15, 20]], return 2.
[[0, 30],[5, 10],[15, 20]]
2