Merge intervals

Given a list of [start, end] intervals, merge all overlapping ones and return them sorted by start. Touching intervals like [1,2] and [2,3] merge.

function solve(intervals)

intervals = [[1,3],[2,6],[8,10],[15,18]]
→ [[1,6],[8,10],[15,18]]

Submit also runs 3 hidden tests, including edge cases.