Calculate Digit Sum of a String

Calculate Digit Sum of a String

What is this problem about?

The Calculate Digit Sum of a String interview question is a recursive string manipulation task. You are given a string s consisting of digits and an integer k. As long as the length of the string is greater than k, you must:

1. Divide the string into groups of size k.
2. For each group, calculate the sum of its digits and replace the group with the string representation of that sum.
3. Concatenate these new strings to form a new version of s. This Calculate Digit Sum of a String coding problem is a simulation of a data reduction process.

Why is this asked in interviews?

Uber uses this to test a candidate's ability to handle string slicing, conversion between characters and integers, and recursive/iterative loops. It evaluates how you manage string building efficiently (to avoid O(N^2) concatenation costs in some languages) and whether you can correctly handle the last group in a string, which might be smaller than k.

Algorithmic pattern used

This utilizes the String, Simulation interview pattern. You use a while loop that continues as long as s.length() > k. Inside, you iterate through the string in steps of k, calculate the sum for each segment, and build the new string.

Example explanation

s = "1111122222", k = 3

1. Groups: "111", "112", "222", "2".
2. Sums: 1+1+1=3, 1+1+2=4, 2+2+2=6, 2=2.
3. New string: "3462".
4. Since "3462" length (4) > k (3), we do it again.
5. Groups: "346", "2". Sums: 13, 2. New string: "132".
6. Final string: "132".

Common mistakes candidates make

• Off-by-one errors: Incorrectly slicing the string, especially for the final group that has fewer than k characters.
• Character to Int conversion: Forgetting that s[i] is a character, and subtracting '0' (or using a parse function) is necessary to get the numeric value.
• Efficiency: Using repeated string concatenation in a loop (which can be O(N^2) in languages like Java or Python) instead of using a StringBuilder or list join.

Interview preparation tip

When a problem asks you to repeat an operation until a condition is met, always double-check your loop's exit condition. Also, be comfortable with string slicing methods (like substring in Java or slice notation in Python) as they are essential for grouping tasks.