Sum of Digits of String After Convert

Sum of Digits of String After Convert

What is this problem about?

In this problem, you are given a string s consisting of lowercase English letters and an integer k.

1. Convert: Replace each letter with its position in the alphabet (e.g., 'a' -> 1, 'b' -> 2, ..., 'z' -> 26). This creates a long string of digits.
2. Transform: Sum the digits of this resulting number. Repeat this transformation k times. The goal is to return the resulting integer after k transformations.

Why is this asked in interviews?

This problem is a favorite for junior-level roles at Microsoft, Google, and Bloomberg. it tests string-to-integer manipulation and the ability to simulate a process multiple times. It requires careful handling of large "numbers" since the initial conversion of a long string can result in a value far too large for a standard 64-bit integer. This forces the candidate to handle the first transformation using strings or by summing as they convert.

Algorithmic pattern used

The pattern is "Iterative Simulation with Digit Summation."

• First step: Iterate through the characters of the string, find their alphabet position, and sum the digits of those positions immediately to avoid creating a massive number.
• Subsequent steps: Take the current sum, extract its digits mathematically (using % 10 and / 10), and calculate a new sum. Repeat this k-1 more times.

Example explanation

String: "zbax", k = 2.

• Conversion: 'z' is 26, 'b' is 2, 'a' is 1, 'x' is 24.
• Concatenated string: "262124".
• Transform 1: Sum digits of "262124" -> 2+6+2+1+2+4 = 17.
• Transform 2: Sum digits of 17 -> 1+7 = 8. Final Result: 8.

Common mistakes candidates make

1. Integer Overflow: Trying to convert the concatenated string into an integer before summing. If the string is long, this will crash the program.
2. Off-by-one in alphabet: Forgetting that 'a' is 1, not 0.
3. K-iterations: Performing the summation k+1 times or k-1 times instead of exactly k.

Interview preparation tip

When you see a problem where you "concatenate" digits to form a number, always be wary of overflow. Usually, you should treat that "number" as a string or sum its digits on the fly. This is a common test of a candidate's practical understanding of data type limits.