L2 · Working engineerSets~2 min · 4 tests#39

Get Unique Characters of a String

Return the set of distinct characters in a Python string and check whether all characters are unique. O(n) time with set() and live tests.

The problem

Return the set of distinct characters in s.

Examples

Example 1

Input

unique_chars('programming')

Expected output

{'m', 'i', 'g', 'p', 'n', 'a', 'o', 'r'}

Example 2
Input
```
unique_chars('aaa')
```
Expected output
```
{'a'}
```

+ 2 hidden tests on Submit — case-sensitive.

Edge cases to ask about

Empty string → set()
Case

Hints

0/3

How an interviewer scores this

0/9

1. Understand the problemRestate it, ask about edge cases and input size.
2. Write a brute-force solutionCorrect first, clever later.
3. Get the correct outputautoAll visible examples pass.
4. Analyse time complexityautoCommit to a Big-O in the Complexity Lab.
5. Analyse space complexityautoExtra memory, not the input.
6. Identify the bottleneckautoMeasure your code — where does it bend?
7. OptimiseUsually: a set or dict instead of a scan.
8. Explain why it worksSay it out loud to Solvi.
9. Test the edge casesautoSubmit passes every hidden test.

Python 3.13 · unique_chars

def unique_chars(s):
    """Return the set of distinct characters in s."""
    # Write your solution here
    pass

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Complexity Lab

What does this cost as n grows?

Interviewers score the analysis as much as the code. Commit to an answer first — then check it, and measure your code against the optimal one at growing input sizes.

Time complexity of the optimal solution

Space complexity (extra memory)

Pick both to reveal the answer.

Measure it

Runs the function on inputs of size 250 up to 16,000 and records the time and peak memory. Slow solutions stop early — a short curve is itself the answer.

From brute force to optimal

The progression an interviewer wants to hear, one step at a time.

Approach	Time	Space	Idea
Manual loop with a set	O(n)	O(k)
bestset(s)	O(n)	O(k)	A string is an iterable of characters.

Walkthrough of the optimal approach (try it yourself first)

set(s) iterates the characters and keeps one of each. A popular follow-up is "are all characters unique?" — len(set(s)) == len(s). Remember that {} is an empty dict; the empty set is set().

Complexity: O(n) time, O(k) space. Each character is hashed once; the set holds k distinct characters.

Reveal the reference solution

def unique_chars(s):
    return set(s)

Follow-ups interviewers ask

Are all characters unique, without extra memory?

Frequently asked interview questions

Core interview concepts, complexities, and follow-ups scored by hiring teams.

What is the time complexity of Get Unique Characters of a String in Python?

The optimal solution runs in O(n) time and O(k) auxiliary space. Each character is hashed once; the set holds k distinct characters.

What is the brute-force approach, and how do you optimise it?

Manual loop with a set: O(n) time, O(k) space. set(s): O(n) time, O(k) space. A string is an iterable of characters.

What follow-up questions do interviewers ask about Get Unique Characters of a String?

Are all characters unique, without extra memory?