Summarize sorted invoice numbers as ranges

A billing report lists the invoice numbers a batch job processed. Long runs of numbers are easier to read as ranges.

Write summarize_ranges(nums) that returns the numbers written as a list of runs.

Input and output:

• nums: a list of up to 10,000 distinct integers in ascending order, each between -1,000,000,000 and 1,000,000,000.
• Returns a list of strings, one per run, in the same order as the numbers.

Rules:

• A run is a longest stretch of consecutive integers, each exactly 1 more than the one before it.
• A run of two or more numbers is written as its first number, then ->, then its last number, with no spaces.
• A run of one number is written as just that number.
• Numbers are written in plain decimal, with a leading - for negatives.
• An empty list returns an empty list.