Add one to a number represented as an array of digits.

Interview question: Plus One

You are given a large integer represented as an integer array `digits`, where each `digits[i]` is the ith digit of the integer. The digits are ordered from most significant to least significant in left-to-right order. The integer does not contain any leading zeros.

Increment the large integer by one and return the resulting array of digits.

## Example
```
Input: digits = [1, 2, 3]
Output: [1, 2, 4]
Explanation: The array represents 123, and 123 + 1 = 124.
```

- The last array element is the ones place, so addition starts there.
- If a digit is less than `9`, adding one does not create a carry.
- In that case, update the digit and return immediately.

Add from the Least Significant Digit

- A digit equal to `9` becomes `0` after adding one and carries to the next digit on the left.
- Continue scanning left while digits are `9`.
- This mirrors elementary addition.

Propagate Carry Through Nines

- If every digit was `9`, the loop turns them all into `0`.
- The result needs a new leading `1`.
- For example, `[9, 9]` becomes `[1, 0, 0]`.

Handle All-Nines Input

- The algorithm mutates the input list in place when possible and only allocates a new leading digit for all-nines cases.
- It scans from right to left once.
- Time complexity is O(n), and extra space complexity is O(1) besides the returned list.