The Code Expander

An easy Python interview practice problem on DataDriven. Write the Python and run it against test cases, with instant feedback.

Domain
Python
Difficulty
easy
Seniority
L3
Asked in
Citadel

Problem

Given an RLE-encoded string where each run is a decimal count followed by a character, return the decoded string. Counts can be multi-digit.

Compressed messages need a decoder to come alive.

Example 1

Input:

encoded = "3a2b4c"

Output:

"aaabbcccc"

Example 2

Input:

encoded = "1a1b1c"

Output:

"abc"

Worked solution and explanation

Why this problem exists in real interviews

This tests string parsing with state tracking: reading digits and characters in sequence to reconstruct the original data. It probes whether a candidate can handle multi-digit numbers and cleanly separate the parsing of counts from characters.

Break down the requirements

Step 1: Scan the string character by character

Accumulate digit characters into a number buffer. When you hit a non-digit, that is the character to repeat.

Step 2: Handle multi-digit counts

Numbers like 12a mean 12 repetitions. You must collect all consecutive digits before treating the next character as the letter.

Step 3: Append the repeated character to the result

Convert the accumulated digits to an integer, repeat the character that many times, and reset the digit buffer.

The solution

State-tracking parser for run-length decoding
def decode(encoded: str) -> str:
    result = ""
    num_buffer = ""
    for ch in encoded:
        if ch.isdigit():
            num_buffer += ch
        else:
            count = int(num_buffer)
            for _ in range(count):
                result += ch
            num_buffer = ""
    return result

Common follow-up questions