Leetcode 253 meeting room II - line sweep algorithm

Leetcode 253 meeting room line sweep algorithm

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.

For example,
Given [[0, 30],[5, 10],[15, 20]],
return 2.
    
// time -> counter
// time -> start -> +1 counter
// time -> finish -> - counter
    
0 -> 1
5 -> 1
10 -> -1
15 -> 1
20 -> -1
30 -> -1
time: O(N logN)  - sorting takes O(nlgN)
space: O(N)
Sweep line approach
https://github.com/pllk/cphb/blob/master/book.pdf (p. 275)
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