L1 · FoundationsText basics~3 min · 5 tests

Capitalise the First Letter of Each Word

Capitalise the first letter of every word in a sentence without title(), lower-casing the rest. String building in Python with split and join, tested.

The problem

Return sentence with the first letter of each word upper-cased and the rest of the word lower-cased. Words are separated by single spaces. Don't use str.title().

Examples

Example 1

Input

capitalize_words('hello world')

Expected output

'Hello World'

Example 2

Input

capitalize_words('pYTHON iS fun')

Expected output

'Python Is Fun'

+ 3 hidden tests on Submit — title() would give It'S.

Edge cases to ask about

Apostrophes
Empty string
Mixed 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 · capitalize_words

def capitalize_words(sentence):
    """Upper-case each word's first letter, lower-case the rest."""
    # 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 read why.

Time complexity of the optimal solution

Space complexity (extra memory)

Pick both to reveal the answer.

From brute force to optimal

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

Approach	Time	Space	Idea
Per-word slice + join	O(n)	O(n)	w[:1] is safe on empty words.

Walkthrough of the optimal approach (try it yourself first)

Split into words, rebuild each as first.upper() + rest.lower(), and join with spaces. w[:1] (not w[0]) avoids an IndexError on empty words.

"it's".title() gives "It'S" because title() starts a new word after the apostrophe — that is why interviewers ask you to write it yourself.

Complexity: O(n) time, O(n) space. Every character is copied once into the new string.

Reveal the reference solution

def capitalize_words(sentence):
    return " ".join(w[:1].upper() + w[1:].lower() for w in sentence.split(" "))

Follow-ups interviewers ask

Keep runs of multiple spaces intact.

Frequently asked interview questions

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

What is the time complexity of Capitalise the First Letter of Each Word in Python?

The optimal solution runs in O(n) time and O(n) auxiliary space. Every character is copied once into the new string.

What follow-up questions do interviewers ask about Capitalise the First Letter of Each Word?

Keep runs of multiple spaces intact.