Calculate Digit Sum of a String
Asked at Capital One
Problem
Calculate Digit Sum of a String repeatedly splits a digit string into groups of k characters, replaces each group with the sum of its digits (written as a string), and joins them — until the string has length at most k. It is a direct simulation that tests careful string handling.
Asked At
| Company | Difficulty | |
|---|---|---|
| Capital One | Easy | View all Capital One questions → |
How to Think About It
Loop while len(s) > k.
Each round: walk the string in steps of k, sum the digits of each chunk, and append that sum as a string (it may have several digits).
The last chunk can be shorter than k — slicing handles that naturally.
Each round shrinks the string substantially (a chunk of k digits becomes at most ceil(log10(9k + 1)) characters), so only a few rounds happen.
Walkthrough: "11111222223", k = 3: 111|112|222|23 -> 3,4,6,5 -> "3465"; then 346|5 -> 13,5 -> "135". Length 3 -> done.
Optimal Approach
Step 1: While len(s) > k:
parts = []
For i in 0, k, 2k, ...: parts.append(str(sum of digits in s[i:i+k])).
s = "".join(parts).
Step 2: Return s.
Time: O(n) per round, and the length shrinks geometrically. Space: O(n).
What Trips People Up in Real Interviews
Stopping when the length is less than k instead of at most k.
Summing the group into a single digit — the sum is written in full (for example 13).
Dropping the final short group.
Converting the whole string to an integer, which overflows for long inputs.
Solution Code
def digitSum(s, k):
while len(s) > k:
parts = []
for i in range(0, len(s), k):
parts.append(str(sum(int(ch) for ch in s[i:i + k])))
s = ''.join(parts)
return sFrequently Asked Questions
What is the Calculate Digit Sum of a String problem?
Calculate Digit Sum of a String repeatedly splits a digit string into groups of `k` characters, replaces each group with the sum of its digits (written as a string), and joins them — until the string has length at most `k`. It is a direct simulation that tests careful string handling.
How do you solve Calculate Digit Sum of a String?
The optimal approach is described in detail above, including step-by-step walkthroughs, complexity analysis, and solution code in Python. Scroll up to the "Optimal Approach" section.
What companies ask Calculate Digit Sum of a String?
Calculate Digit Sum of a String is asked at Capital One. It is a easy difficulty problem.
What are common mistakes on Calculate Digit Sum of a String?
- Stopping when the length is less than `k` instead of at most `k`.
- Summing the group into a single digit — the sum is written in full (for example `13`).
- Dropping the final short group.
- Converting the whole string to an integer, which overflows for long inputs.