This problem was asked by Facebook.
Given a sorted list of numbers, return a list of strings that represent all of the consecutive numbers.
Assume that all numbers will be greater than or equal to 0, and each element can repeat.
Input: [0, 1, 2, 5, 7, 8, 9, 9, 10, 11, 15]
Output: ['0->2', '5->5', '7->11', '15->15']