Given a collection of intervals, merge all overlapping intervals.
For example:
Given [1,3],[2,6],[8,10],[15,18]
return [1,6],[8,10],[15,18]

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example:
Given binary tree [3,9,20,null,null,15,7],
    3
   / \
  9  20
    /  \
   15   7
return its level order traversal as:
[
  [3],
  [9,20],
  [15,7]
]