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This is a medium difficulty Coding & Algorithms question, commonly asked during Onsite rounds at Coinbase.

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This question is commonly asked for Data Scientist candidates at Coinbase during technical interviews.

Implement Plus One | Coinbase Coding Question

Implement Plus One

Company: Coinbase

Role: Data Scientist

Category: Coding & Algorithms

Difficulty: medium

Interview Round: Onsite

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.

Quick Answer: This question evaluates proficiency in array manipulation, elementary arithmetic operations such as carry propagation, and careful handling of edge cases while meeting time and space complexity constraints.

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]` Aim for `O(n)` time and `O(1)` extra space apart from the returned array.

Constraints

1 <= digits.length
0 <= digits[i] <= 9
digits has no leading zeros unless the integer is exactly 0

Examples

Input: ([1, 2, 3],)

Expected Output: [1, 2, 4]

Explanation: No carry needed; the last digit 3 becomes 4.

Input: ([4, 3, 2, 1],)

Expected Output: [4, 3, 2, 2]

Explanation: The last digit 1 becomes 2; no carry propagates.

Input: ([9, 9, 9],)

Expected Output: [1, 0, 0, 0]

Explanation: Every digit is 9, so each becomes 0 and the carry propagates past the front, prepending a new leading 1.

Input: ([0],)

Expected Output: [1]

Explanation: Single zero plus one is one.

Input: ([9],)

Expected Output: [1, 0]

Explanation: Single nine carries out, producing a two-digit result [1, 0].

Input: ([1, 9, 9],)

Expected Output: [2, 0, 0]

Explanation: The two trailing nines roll to zero and the carry increments the leading 1 to 2.

Hints

Adding one only affects the trailing digits while there is a carry. Walk the array from the least significant (rightmost) digit toward the most significant.
If a digit is less than 9, increment it and return immediately — no further carry is possible.
If a digit is 9, set it to 0 and continue the carry leftward. If the carry propagates past the first digit (all nines), prepend a 1 to the result, e.g. [9,9,9] -> [1,0,0,0].

Quick Overview

This question evaluates proficiency in array manipulation, elementary arithmetic operations such as carry propagation, and careful handling of edge cases while meeting time and space complexity constraints.