Alternating Digit Sum

Alternating Digit Sum

What is this problem about?

The "Alternating Digit Sum interview question" is a numeric manipulation problem. Given a positive integer n, you need to calculate the sum of its digits, but with a catch: the signs of the digits alternate. The first (most significant) digit is positive, the second is negative, the third is positive, and so on.

Why is this asked in interviews?

Companies like Amazon and eBay use the "Alternating Digit Sum coding problem" as an introductory technical question. it tests a candidate's ability to process digits of a number, handle basic mathematical logic, and manage parity (even/odd positions). It's a test of clean coding and basic algorithmic thinking.

Algorithmic pattern used

This problem uses Digit Extraction and Parity Tracking.

1. Conversion: Convert the number into a string or a list of digits so you can easily access them from left to right.
2. Looping: Iterate through the digits.
3. Sign Logic: Use a multiplier that flips between +1 and -1 for each step, or check if the current index is even or odd.
4. Accumulation: Add the result of (digit * sign) to a running total.

Example explanation

Input: 521

1. First digit: 5 (Positive) -> Total = 5.
2. Second digit: 2 (Negative) -> Total = 5 - 2 = 3.
3. Third digit: 1 (Positive) -> Total = 3 + 1 = 4. Result: 4. Input: 10
4. First digit: 1 (Positive) -> Total = 1.
5. Second digit: 0 (Negative) -> Total = 1 - 0 = 1. Result: 1.

Common mistakes candidates make

• Processing in reverse: Extracting digits using % 10 and / 10 gives them in reverse order (least significant first). If you do this, you must adjust the starting sign based on whether the total number of digits is even or odd.
• Off-by-one errors: Starting with a negative sign for the first digit instead of a positive one.
• String conversion overhead: While converting to a string is easy, some interviewers might prefer the purely mathematical approach (using logarithms or counting digits first).

Interview preparation tip

Get comfortable with both ways of processing digits: converting the number to a string for easy indexing, and using the modulo/division math for memory efficiency. Know how to calculate the number of digits in an integer without a loop (using log10).