Plus One

An odometer stores a non-negative whole number as a list of its decimal digits, most significant digit first. The number has no leading zeros, except that zero itself is [0].

Write add_one(digits) -> list that returns the digits of the number plus one, in the same format.

add_one([1, 2, 3])   # [1, 2, 4]
add_one([4, 9, 9])   # [5, 0, 0]
add_one([9, 9])      # [1, 0, 0]
add_one([0])         # [1]

• 1 <= len(digits) <= 10^5, each digit is in 0..9.
• The number can be far too long for a machine integer, so work with the digits directly instead of converting the whole list to a number.
• Aim for O(n) time.