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easyBasicsSometimes asked

Capitalize Each Word

Basics · 6 min

Given a sentence s, return it with the first letter of every word capitalised and the rest unchanged.

In plain English

Capitalise a letter when it is the first character or follows a space.

Examples

Example 1

Input: s = "hello world"
Output: "Hello World"
Note: Each word's first letter upper.

Example 2

Input: s = "a b c"
Output: "A B C"
Note: Singles capitalised.

Example 3

Input: s = "hi"
Output: "Hi"
Note: First letter.

Constraints

0 ≤ s.length ≤ 10^4

Clarifying questions1First question: Lower-case the rest of each word too?Explore questions

Q1.Lower-case the rest of each word too?
This version leaves the rest unchanged; title-case would lower it.

Quick interview revision

Understand

30-second explanation

Build the interview-ready mental model before comparing implementations.

Walk the string tracking whether the previous character was a space (or it's the start). At a word-start letter, upper-case it; otherwise copy as-is.

Interview contextWhat is testedWhere candidates slipHow to stand outExplore interview context

What's being tested

Whether you track word boundaries with a flag and capitalise selectively.

Where candidates slip

Missing the first word, or upper-casing the whole string.

How to stand out

Handle leading spaces and multiple spaces cleanly via the flag.

Choose

Compare the available approaches

Use constraints and trade-offs to select the route you can defend in an interview.

Learn & trace

Build each solution step by step

Move from the core idea to a visual trace, implementation, proof, and edge cases.

03.1

Approach 1 of 1 · Optimal

Word-start uppercase

TimeO(n)SpaceO(n)

This is the standard, expected solution — lead with it.

Stuck? Progressive hintsReveal one at a time

1Hint 1Explore hint

When does a letter begin a word?

2Hint 2Explore hint

Track a 'start of word' boolean, set after each space.

3Hint 3Explore hint

Capitalise only word-start letters; leave the rest alone.

A · Core idea

Capitalise the first letter of each word: a letter is a word-start when it is the first character or the previous character was a space. Upper-case exactly those letters and copy the rest unchanged.

Why a 'word-start' flag. A single boolean — 'is the next letter the start of a word?' — set true initially and after each space, tells you which letters to capitalise. It cleanly handles the first word (no leading space) and runs of spaces.

Why not upper-case everything. That's a different problem (to-uppercase). Here only the leading letter of each word changes; the rest of each word is left as-is.

Edge handling. Reset the flag to true on every space; clear it once you've processed a non-space. Non-letter word-starts (e.g. a leading digit) simply aren't shifted.

B · Visual dry run

Interactive dry run

Select a frame to connect the decision with its visual state.

s = "hi yo"

start → H

StateH

Final result"Hi Yo"

C · Code with teaching comments

solution.javaJava

Word-start uppercase — optimal

1 teaching note

1
public String capitalize(String s) {
2
    char[] c = s.toCharArray();
3
    boolean start = true;
4
    for (int i = 0; i < c.length; i++) {
5
        if (c[i] == ' ') {
6
            start = true;
7
        } else {
8
            // Upper-case the first letter of each word.
9
            if (start && c[i] >= 'a' && c[i] <= 'z') c[i] -= 32;
10
            start = false;
11
        }
12
    }
13
    return new String(c);
14
}

Yellow row = the algorithm's core logic. Amber `//` lines above it = what you'd say out loud.14 lines

D · Why it works and complexityO(n) timeO(n) spaceOpen the invariant and proofExplore the proof

ComplexityO(n) time, O(n) space — one character visit and a flag update each step.

E · Edge cases and pitfalls5s = "hello world" — → "Hello World".Explore edge cases

Edge cases handled

s = "hello world"→ "Hello World".
s = "a b c"→ "A B C".
s = "hi"First word only → "Hi".

Pitfalls

Capitalising every letter (that's to-uppercase).
Forgetting the very first word when there's no leading space.

Practice

Try the solution yourself

Edit the optimal implementation, test another input, and observe the result.

Editable code previewpublic String capitalize(String s) { char[] c = s.toCharArray(); boolean start = true; for (int i = 0; i < c.length; i++) { if (c[i] == ' ') {Explore the playground

Try it — Java· run a complete program; method snippets need a driver

Avoid mistakes

Recognize the interview traps

Understand why common attempts fail before reviewing the corrected implementation.

Capitalising every letter (that's to-uppercase).
Forgetting the very first word when there's no leading space.

Revise

Carry the pattern into the interview

Quick interview revision

Pattern to remember

Word-start flag

Code pattern to memorize

start=true; for ch: if ch==' ': start=true else: if start upper(ch); start=false

Rules to remember

1Word start = first char or after space.
2Upper-case only those.
3Reset flag on spaces.

Use this pattern when

Per-word boundary processing

Interview takeaway

A 'just saw a space' flag marks where words begin.

Continue

Transfer the pattern to another problem

Use a direct variation or pattern sibling to prove the idea is reusable.

Transferable patternA core fresher pattern: Word-start flag. Master it on small inputs and it transfers to harder problems.

Practice queueDirect variations and pattern siblings

Direct follow-ups

Convert to Uppercase
Capitalises every letter, not just word starts.

easyBasicsSometimes asked

Capitalize Each Word

Basics · 6 min

Given a sentence s, return it with the first letter of every word capitalised and the rest unchanged.

In plain English

Capitalise a letter when it is the first character or follows a space.

Examples

Example 1

Input: s = "hello world"
Output: "Hello World"
Note: Each word's first letter upper.

Example 2

Input: s = "a b c"
Output: "A B C"
Note: Singles capitalised.

Example 3

Input: s = "hi"
Output: "Hi"
Note: First letter.

Constraints

0 ≤ s.length ≤ 10^4

Clarifying questions1First question: Lower-case the rest of each word too?Explore questions

Q1.Lower-case the rest of each word too?
This version leaves the rest unchanged; title-case would lower it.

Quick interview revision

Understand

30-second explanation

Build the interview-ready mental model before comparing implementations.

Walk the string tracking whether the previous character was a space (or it's the start). At a word-start letter, upper-case it; otherwise copy as-is.

Interview contextWhat is testedWhere candidates slipHow to stand outExplore interview context

What's being tested

Whether you track word boundaries with a flag and capitalise selectively.

Where candidates slip

Missing the first word, or upper-casing the whole string.

How to stand out

Handle leading spaces and multiple spaces cleanly via the flag.

Choose

Compare the available approaches

Use constraints and trade-offs to select the route you can defend in an interview.

Learn & trace

Build each solution step by step

Move from the core idea to a visual trace, implementation, proof, and edge cases.

03.1

Approach 1 of 1 · Optimal

Word-start uppercase

TimeO(n)SpaceO(n)

This is the standard, expected solution — lead with it.

Stuck? Progressive hintsReveal one at a time

1Hint 1Explore hint

When does a letter begin a word?

2Hint 2Explore hint

Track a 'start of word' boolean, set after each space.

3Hint 3Explore hint

Capitalise only word-start letters; leave the rest alone.

A · Core idea

Capitalise the first letter of each word: a letter is a word-start when it is the first character or the previous character was a space. Upper-case exactly those letters and copy the rest unchanged.

Why not upper-case everything. That's a different problem (to-uppercase). Here only the leading letter of each word changes; the rest of each word is left as-is.

Edge handling. Reset the flag to true on every space; clear it once you've processed a non-space. Non-letter word-starts (e.g. a leading digit) simply aren't shifted.

B · Visual dry run

Interactive dry run

Select a frame to connect the decision with its visual state.

s = "hi yo"

start → H

StateH

Final result"Hi Yo"

C · Code with teaching comments

solution.javaJava

Word-start uppercase — optimal

1 teaching note

1
public String capitalize(String s) {
2
    char[] c = s.toCharArray();
3
    boolean start = true;
4
    for (int i = 0; i < c.length; i++) {
5
        if (c[i] == ' ') {
6
            start = true;
7
        } else {
8
            // Upper-case the first letter of each word.
9
            if (start && c[i] >= 'a' && c[i] <= 'z') c[i] -= 32;
10
            start = false;
11
        }
12
    }
13
    return new String(c);
14
}

Yellow row = the algorithm's core logic. Amber `//` lines above it = what you'd say out loud.14 lines

D · Why it works and complexityO(n) timeO(n) spaceOpen the invariant and proofExplore the proof

ComplexityO(n) time, O(n) space — one character visit and a flag update each step.

E · Edge cases and pitfalls5s = "hello world" — → "Hello World".Explore edge cases

Edge cases handled

s = "hello world"→ "Hello World".
s = "a b c"→ "A B C".
s = "hi"First word only → "Hi".

Pitfalls

Capitalising every letter (that's to-uppercase).
Forgetting the very first word when there's no leading space.

Practice

Try the solution yourself

Edit the optimal implementation, test another input, and observe the result.

Editable code previewpublic String capitalize(String s) { char[] c = s.toCharArray(); boolean start = true; for (int i = 0; i < c.length; i++) { if (c[i] == ' ') {Explore the playground

Try it — Java· run a complete program; method snippets need a driver

Avoid mistakes

Recognize the interview traps

Understand why common attempts fail before reviewing the corrected implementation.

Capitalising every letter (that's to-uppercase).
Forgetting the very first word when there's no leading space.

Revise

Carry the pattern into the interview

Quick interview revision

Pattern to remember

Word-start flag

Code pattern to memorize

start=true; for ch: if ch==' ': start=true else: if start upper(ch); start=false

Rules to remember

1Word start = first char or after space.
2Upper-case only those.
3Reset flag on spaces.

Use this pattern when

Per-word boundary processing

Interview takeaway

A 'just saw a space' flag marks where words begin.

Continue

Transfer the pattern to another problem

Use a direct variation or pattern sibling to prove the idea is reusable.

Transferable patternA core fresher pattern: Word-start flag. Master it on small inputs and it transfers to harder problems.

Practice queueDirect variations and pattern siblings

Direct follow-ups

Convert to Uppercase
Capitalises every letter, not just word starts.