The Solo Act

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

Problem

Given a list of integers, return a list of the values that appear exactly once, preserving input order.

One-and-done values only.

Example 1

Input:

codes = [200,404,200,500]

Output:

[404,500]

Example 2

Input:

codes = [1,2,3,2,1]

Output:

[3]

Worked solution and explanation

Why this problem exists in real interviews

This tests frequency counting followed by order-preserving filtering. It is a two-pass pattern: first count occurrences, then filter to elements that appeared exactly once while maintaining original order.

Break down the requirements

Step 1: Count occurrences of each code

Build a frequency dict in one pass over the list.

Step 2: Filter to codes with count exactly 1, preserving order

Iterate the original list a second time, collecting codes whose count is 1.

The solution

Two-pass frequency filter
def find_singletons(codes: list) -> list:
    counts = {}
    for code in codes:
        counts[code] = counts.get(code, 0) + 1
    result = []
    for code in codes:
        if counts[code] == 1:
            result.append(code)
    return result

Common follow-up questions