Write a function called sum_of_intervals() that accepts an array of
intervals, and returns the sum of all the interval lengths. Overlapping
intervals should only be counted once.
Intervals are represented by a pair of integers in the form of an array. The
first value of the interval will always be less than the second value. Interval
example: (1, 5) is an interval from 1 to 5. The length of this interval is 4.
List containing overlapping intervals:
[
(1,4),
(7, 10),
(3, 5)
]
The sum of the lengths of these intervals is 7. Since (1, 4) and (3, 5)
overlap, we can treat the interval as (1, 5), which has a length of 4.
sumIntervals([(1,2), (6, 10), (11, 15)]); // => 9
sumIntervals([(1,4), (7, 10), (3, 5)]); // => 7
sumIntervals([(1,5), (10, 20), (1, 6), (16, 19), (5, 11)]); // => 19