Add Two Numbers:
You are given two linked lists representing two non-negative numbers.
The digits are stored in reverse order and each of their nodes contain a
single digit. Add the two numbers and return it as a linked list.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Find the k largest elements group in an unsorted array of length n