Implement Plus One

Given an integer represented as an array of digits `digits`, where the most significant digit comes first, return the array representing the integer plus one. Assume: - `digits` contains only values from `0` to `9` - the integer has no leading zeros unless it is exactly zero Examples: - `[1, 2, 3] -> [1, 2, 4]` - `[4, 3, 2, 1] -> [4, 3, 2, 2]` - `[9, 9, 9] -> [1, 0, 0, 0]` Implement the solution in Python. Aim for `O(n)` time and `O(1)` extra space apart from the returned array.