Loading…

easyBasicsSometimes asked

Reverse Words in a Sentence

Basics · 6 min

Given a sentence s, return it with the order of the words reversed (single spaces between words, no leading/trailing spaces in the output).

In plain English

Split into words, reverse the list, and join with single spaces.

Examples

Example 1

Input: s = "the quick fox"
Output: "fox quick the"
Note: Word order reversed.

Example 2

Input: s = "hello"
Output: "hello"
Note: One word.

Example 3

Input: s = "a b c"
Output: "c b a"
Note: Reversed.

Constraints

1 ≤ s.length ≤ 10^4

Clarifying questions1First question: Collapse multiple spaces in output?Explore questions

Q1.Collapse multiple spaces in output?
Yes — I join with single spaces and trim.

Quick interview revision

Understand

30-second explanation

Build the interview-ready mental model before comparing implementations.

Split on whitespace (dropping empties), reverse the array of words, and join with a single space.

Interview contextWhat is testedWhere candidates slipHow to stand outExplore interview context

What's being tested

Whether you operate at word granularity and normalise messy spacing.

Where candidates slip

Character reversal, or empty tokens from a bad split.

How to stand out

Mention the in-place O(1)-space version: reverse whole, then reverse each word.

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

Split, reverse, join

TimeO(n)SpaceO(n)

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

Stuck? Progressive hintsReveal one at a time

1Hint 1Explore hint

Reverse words, not characters — what's the unit you operate on?

2Hint 2Explore hint

Split on whitespace, reverse the list, join with single spaces.

3Hint 3Explore hint

Trim and split on \s+ so extra spaces don't make empty tokens.

A · Core idea

Reverse the order of words (not characters): tokenise on whitespace, reverse the list of words, and join with single spaces.

Why tokenise → reverse → join. Splitting on runs of whitespace turns the sentence into clean word tokens (dropping extra spaces); reversing the token list flips word order; joining with one space normalises the output spacing. Each step is simple and composes cleanly.

Why `trim()` + split on `\s+`. Leading/trailing spaces would otherwise create empty tokens; trimming and splitting on whitespace runs avoids them so the output has no stray spaces.

Don't reverse characters. Reversing the characters of the whole string would also reverse each word's spelling — a different (wrong) result. The unit of reversal here is the word.

B · Visual dry run

Interactive dry run

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

s = "the quick fox"

split

[the, quick, fox]

Statetokens

Final result"fox quick the"

C · Code with teaching comments

solution.javaJava

Split, reverse, join — optimal

2 teaching notes

1
public String reverseWords(String s) {
2
    String[] words = s.trim().split("\\s+");
3
    StringBuilder sb = new StringBuilder();
4
    for (int i = words.length - 1; i >= 0; i--) {
5
        sb.append(words[i]);
6
        if (i > 0) sb.append(' ');
7
    }
8
    return sb.toString();
9
}

Yellow row = the algorithm's core logic. Amber `//` lines above it = what you'd say out loud.9 lines

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

ComplexityO(n) time, O(n) space — tokenising, reversing, and joining are each linear in the input length.

E · Edge cases and pitfalls5s = "hello" — One word → "hello".Explore edge cases

Edge cases handled

s = "hello"One word → "hello".
s = " a b "Extra spaces normalised → "b a".
s = "a b c"→ "c b a".

Pitfalls

Reversing characters instead of words.
Leaving double spaces from a naive split.

Practice

Try the solution yourself

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

Editable code previewpublic String reverseWords(String s) { String[] words = s.trim().split("\\s+"); StringBuilder sb = new StringBuilder(); for (int i = words.length - 1; i >= 0; i--) { sb.append(words[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.

Reversing characters instead of words.
Leaving double spaces from a naive split.

Revise

Carry the pattern into the interview

Quick interview revision

Pattern to remember

Tokenise → reverse → join

Code pattern to memorize

' '.join(s.split()[::-1])

Rules to remember

1Split on \s+.
2Reverse the word list.
3Join with single spaces.

Use this pattern when

Word-order manipulation

Interview takeaway

Word-level operations start by tokenising into words.

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: Tokenise → reverse → join. Master it on small inputs and it transfers to harder problems.

Practice queueDirect variations and pattern siblings

Direct follow-ups

Reverse Words in a String
Coming soon
In-place: reverse all, then each word.
Reverse a String
Character-level reversal.

Same pattern

easyBasicsSometimes asked

Reverse Words in a Sentence

Basics · 6 min

Given a sentence s, return it with the order of the words reversed (single spaces between words, no leading/trailing spaces in the output).

In plain English

Split into words, reverse the list, and join with single spaces.

Examples

Example 1

Input: s = "the quick fox"
Output: "fox quick the"
Note: Word order reversed.

Example 2

Input: s = "hello"
Output: "hello"
Note: One word.

Example 3

Input: s = "a b c"
Output: "c b a"
Note: Reversed.

Constraints

1 ≤ s.length ≤ 10^4

Clarifying questions1First question: Collapse multiple spaces in output?Explore questions

Q1.Collapse multiple spaces in output?
Yes — I join with single spaces and trim.

Quick interview revision

Understand

30-second explanation

Build the interview-ready mental model before comparing implementations.

Split on whitespace (dropping empties), reverse the array of words, and join with a single space.

Interview contextWhat is testedWhere candidates slipHow to stand outExplore interview context

What's being tested

Whether you operate at word granularity and normalise messy spacing.

Where candidates slip

Character reversal, or empty tokens from a bad split.

How to stand out

Mention the in-place O(1)-space version: reverse whole, then reverse each word.

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

Split, reverse, join

TimeO(n)SpaceO(n)

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

Stuck? Progressive hintsReveal one at a time

1Hint 1Explore hint

Reverse words, not characters — what's the unit you operate on?

2Hint 2Explore hint

Split on whitespace, reverse the list, join with single spaces.

3Hint 3Explore hint

Trim and split on \s+ so extra spaces don't make empty tokens.

A · Core idea

Reverse the order of words (not characters): tokenise on whitespace, reverse the list of words, and join with single spaces.

Why `trim()` + split on `\s+`. Leading/trailing spaces would otherwise create empty tokens; trimming and splitting on whitespace runs avoids them so the output has no stray spaces.

Don't reverse characters. Reversing the characters of the whole string would also reverse each word's spelling — a different (wrong) result. The unit of reversal here is the word.

B · Visual dry run

Interactive dry run

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

s = "the quick fox"

split

[the, quick, fox]

Statetokens

Final result"fox quick the"

C · Code with teaching comments

solution.javaJava

Split, reverse, join — optimal

2 teaching notes

1
public String reverseWords(String s) {
2
    String[] words = s.trim().split("\\s+");
3
    StringBuilder sb = new StringBuilder();
4
    for (int i = words.length - 1; i >= 0; i--) {
5
        sb.append(words[i]);
6
        if (i > 0) sb.append(' ');
7
    }
8
    return sb.toString();
9
}

Yellow row = the algorithm's core logic. Amber `//` lines above it = what you'd say out loud.9 lines

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

ComplexityO(n) time, O(n) space — tokenising, reversing, and joining are each linear in the input length.

E · Edge cases and pitfalls5s = "hello" — One word → "hello".Explore edge cases

Edge cases handled

s = "hello"One word → "hello".
s = " a b "Extra spaces normalised → "b a".
s = "a b c"→ "c b a".

Pitfalls

Reversing characters instead of words.
Leaving double spaces from a naive split.

Practice

Try the solution yourself

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

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.

Reversing characters instead of words.
Leaving double spaces from a naive split.

Revise

Carry the pattern into the interview

Quick interview revision

Pattern to remember

Tokenise → reverse → join

Code pattern to memorize

' '.join(s.split()[::-1])

Rules to remember

1Split on \s+.
2Reverse the word list.
3Join with single spaces.

Use this pattern when

Word-order manipulation

Interview takeaway

Word-level operations start by tokenising into words.

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: Tokenise → reverse → join. Master it on small inputs and it transfers to harder problems.

Practice queueDirect variations and pattern siblings

Direct follow-ups

Reverse Words in a String
Coming soon
In-place: reverse all, then each word.
Reverse a String
Character-level reversal.

Same pattern