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easyBasicsFrequently asked

Reverse a String

Basics · 6 min

Given a string s, return it with the characters in reverse order.

In plain English

Either swap from both ends inward (in place on a char array) or build the result back-to-front.

Examples

Example 1

Input: s = "hello"
Output: "olleh"
Note: Characters reversed.

Example 2

Input: s = "a"
Output: "a"
Note: Single char unchanged.

Example 3

Input: s = "ab"
Output: "ba"
Note: Two chars swapped.

Constraints

0 ≤ s.length ≤ 10^4

Clarifying questions1First question: Reverse by code unit or grapheme?Explore questions

Q1.Reverse by code unit or grapheme?
Standard problems reverse code units; emoji/combining marks need care.

Quick interview revision

Understand

30-second explanation

Build the interview-ready mental model before comparing implementations.

Convert to a char array and two-pointer swap the ends inward, or in Python use slicing `s[::-1]`. O(n) time.

Interview contextWhat is testedWhere candidates slipHow to stand outExplore interview context

What's being tested

Whether you know strings are immutable and reverse in place on a char array.

Where candidates slip

Mutating the String, or O(n²) prepend-building.

How to stand out

Mention Unicode/surrogate pairs as a caveat for true character reversal.

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

Two-pointer swap

TimeO(n)SpaceO(1) extra (in place on char array)

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

Stuck? Progressive hintsReveal one at a time

1Hint 1Explore hint

Reuse the array-reverse idea — on characters this time.

2Hint 2Explore hint

Java strings are immutable: work on a char array.

3Hint 3Explore hint

Two pointers from both ends, swap, converge; Python can use s[::-1].

A · Core idea

Reversing a string is the array two-pointer swap applied to characters: swap the first and last, second and second-last, converging in the middle. O(n) time, O(1) extra space.

Why a char array in Java. Java Strings are immutable — you can't assign s[i] = …. So you convert to a char[], swap in place, and build a new String. Python strings are also immutable, but slicing s[::-1] builds the reversed copy in one expression.

Why two pointers, not a fresh buffer per char. The in-place swap touches each character once with no per-character allocation. Building the result by repeatedly prepending characters would be O(n²) in many languages.

Loop bound. Use left < right so the middle character of an odd-length string is left untouched and the pointers don't cross and undo the work.

B · Visual dry run

Interactive dry run

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

s = "abcd"

l=0,r=3

swap a,d

Statedbca

Final result"dcba"

C · Code with teaching comments

solution.javaJava

Two-pointer swap — optimal

2 teaching notes

1
public String reverse(String s) {
2
    // Java strings are immutable, so reverse a mutable char array.
3
    char[] c = s.toCharArray();
4
    int left = 0, right = c.length - 1;
5
    while (left < right) {
6
        char t = c[left];
7
        c[left] = c[right];
8
        c[right] = t;
9
        left++;
10
        right--;
11
    }
12
    return new String(c);
13
}

Yellow row = the algorithm's core logic. Amber `//` lines above it = what you'd say out loud.13 lines

D · Why it works and complexityO(n) timeO(1) extra (in place on char array) spaceOpen the invariant and proofExplore the proof

Mentioning that Java strings are immutable (hence the char array) shows language awareness.

ComplexityO(n) time, O(1) extra (in place on char array) space — n/2 swaps with O(1) extra state (the output buffer aside).

E · Edge cases and pitfalls5s = "a" — Single char unchanged.Explore edge cases

Edge cases handled

s = "a"Single char unchanged.
s = ""Empty string unchanged.
s = "ab"Two chars swap to "ba".

Pitfalls

Trying to mutate a Java String directly (immutable).
Off-by-one on the right index (length vs length − 1).

Practice

Try the solution yourself

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

Editable code previewpublic String reverse(String s) { char[] c = s.toCharArray(); int left = 0, right = c.length - 1; while (left < right) { char t = c[left];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.

Trying to mutate a Java String directly (it's immutable).
Off-by-one on the right index.

Revise

Carry the pattern into the interview

Quick interview revision

Pattern to remember

Two-pointer swap (chars)

Code pattern to memorize

char[] c; l,r=0,n-1; while l<r: swap; l++; r--

Rules to remember

1Work on a char array in Java.
2Swap ends, converge.
3Python: s[::-1].

Use this pattern when

In-place reversal of a sequence

Interview takeaway

String reverse is array reverse on characters.

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: Two-pointer swap (chars). Master it on small inputs and it transfers to harder problems.

Practice queueDirect variations and pattern siblings

Direct follow-ups

Same pattern

easyBasicsFrequently asked

Reverse a String

Basics · 6 min

Given a string s, return it with the characters in reverse order.

In plain English

Either swap from both ends inward (in place on a char array) or build the result back-to-front.

Examples

Example 1

Input: s = "hello"
Output: "olleh"
Note: Characters reversed.

Example 2

Input: s = "a"
Output: "a"
Note: Single char unchanged.

Example 3

Input: s = "ab"
Output: "ba"
Note: Two chars swapped.

Constraints

0 ≤ s.length ≤ 10^4

Clarifying questions1First question: Reverse by code unit or grapheme?Explore questions

Q1.Reverse by code unit or grapheme?
Standard problems reverse code units; emoji/combining marks need care.

Quick interview revision

Understand

30-second explanation

Build the interview-ready mental model before comparing implementations.

Convert to a char array and two-pointer swap the ends inward, or in Python use slicing `s[::-1]`. O(n) time.

Interview contextWhat is testedWhere candidates slipHow to stand outExplore interview context

What's being tested

Whether you know strings are immutable and reverse in place on a char array.

Where candidates slip

Mutating the String, or O(n²) prepend-building.

How to stand out

Mention Unicode/surrogate pairs as a caveat for true character reversal.

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

Two-pointer swap

TimeO(n)SpaceO(1) extra (in place on char array)

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

Stuck? Progressive hintsReveal one at a time

1Hint 1Explore hint

Reuse the array-reverse idea — on characters this time.

2Hint 2Explore hint

Java strings are immutable: work on a char array.

3Hint 3Explore hint

Two pointers from both ends, swap, converge; Python can use s[::-1].

A · Core idea

Reversing a string is the array two-pointer swap applied to characters: swap the first and last, second and second-last, converging in the middle. O(n) time, O(1) extra space.

Loop bound. Use left < right so the middle character of an odd-length string is left untouched and the pointers don't cross and undo the work.

B · Visual dry run

Interactive dry run

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

s = "abcd"

l=0,r=3

swap a,d

Statedbca

Final result"dcba"

C · Code with teaching comments

solution.javaJava

Two-pointer swap — optimal

2 teaching notes

1
public String reverse(String s) {
2
    // Java strings are immutable, so reverse a mutable char array.
3
    char[] c = s.toCharArray();
4
    int left = 0, right = c.length - 1;
5
    while (left < right) {
6
        char t = c[left];
7
        c[left] = c[right];
8
        c[right] = t;
9
        left++;
10
        right--;
11
    }
12
    return new String(c);
13
}

Yellow row = the algorithm's core logic. Amber `//` lines above it = what you'd say out loud.13 lines

D · Why it works and complexityO(n) timeO(1) extra (in place on char array) spaceOpen the invariant and proofExplore the proof

Mentioning that Java strings are immutable (hence the char array) shows language awareness.

ComplexityO(n) time, O(1) extra (in place on char array) space — n/2 swaps with O(1) extra state (the output buffer aside).

E · Edge cases and pitfalls5s = "a" — Single char unchanged.Explore edge cases

Edge cases handled

s = "a"Single char unchanged.
s = ""Empty string unchanged.
s = "ab"Two chars swap to "ba".

Pitfalls

Trying to mutate a Java String directly (immutable).
Off-by-one on the right index (length vs length − 1).

Practice

Try the solution yourself

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

Editable code previewpublic String reverse(String s) { char[] c = s.toCharArray(); int left = 0, right = c.length - 1; while (left < right) { char t = c[left];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.

Trying to mutate a Java String directly (it's immutable).
Off-by-one on the right index.

Revise

Carry the pattern into the interview

Quick interview revision

Pattern to remember

Two-pointer swap (chars)

Code pattern to memorize

char[] c; l,r=0,n-1; while l<r: swap; l++; r--

Rules to remember

1Work on a char array in Java.
2Swap ends, converge.
3Python: s[::-1].

Use this pattern when

In-place reversal of a sequence

Interview takeaway

String reverse is array reverse on characters.

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: Two-pointer swap (chars). Master it on small inputs and it transfers to harder problems.

Practice queueDirect variations and pattern siblings

Direct follow-ups

Same pattern