String Reversal Approaches in DSA Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the simplest way to reverse a string using built-in Java methods?

Use StringBuilder or StringBuffer with the reverse() method. For example: new StringBuilder(str).reverse().toString().

beginner

How can you reverse a string manually using a character array in Java?

Convert the string to a char array, then swap characters from start and end moving towards the center until all are reversed.

beginner

What is the time complexity of reversing a string by swapping characters in place?

The time complexity is O(n), where n is the length of the string, because each character is visited once.

intermediate

Why is using recursion to reverse a string less efficient in Java?

Recursion uses extra memory on the call stack for each character, which can lead to stack overflow for large strings and is less efficient than iterative methods.

intermediate

Explain the difference between StringBuilder and manual reversal using a char array.

StringBuilder.reverse() is a built-in method that reverses the string efficiently and concisely. Manual reversal requires explicit swapping of characters in a char array, giving more control but needing more code.

Which Java class provides a built-in method to reverse strings easily?

A StringBuilder

B String

C Arrays

D Character

StringBuilder has the reverse() method that reverses the string content.

What is the first step to reverse a string manually in Java?

A Use substring method

B Convert the string to a char array

C Call reverse() on the string

D Use recursion directly on the string

You need to convert the string to a char array to swap characters manually.

What is the time complexity of reversing a string by swapping characters?

A O(1)

B O(n^2)

C O(log n)

D O(n)

Each character is visited once, so the complexity is linear, O(n).

Why might recursion be a poor choice for reversing very long strings?

A It is faster than iteration

B It does not work on strings

C It uses extra stack memory and can cause stack overflow

D It changes the original string permanently

Recursion uses stack frames for each call, risking stack overflow on large inputs.

Which method is more concise for reversing a string in Java?

A Using StringBuilder.reverse()

B Manually swapping characters in a char array

C Using recursion

D Using substring concatenation

StringBuilder.reverse() is a single method call and concise.

Describe two different ways to reverse a string in Java and their pros and cons.

Think about built-in methods vs manual approaches and memory usage.

Explain why reversing a string by swapping characters has O(n) time complexity.

Consider how many times each character is processed.

Practice

1. Which of the following methods correctly reverses a string in Java using built-in functions?

easy

A. Using new StringBuilder(str).reverse().toString()

B. Using str.reverse()

C. Using str.toCharArray().reverse()

D. Using String.reverse(str)

Solution

1. Step 1: Understand Java String reversal methods

   Java's String class does not have a reverse() method, but StringBuilder does.

2. Step 2: Identify correct usage of StringBuilder

   Creating a StringBuilder from the string and calling reverse() then toString() returns the reversed string.

3. Final Answer:

   Using new StringBuilder(str).reverse().toString() -> Option A

4. Quick Check:

   StringBuilder.reverse() = correct reversal [OK]

Hint: Use StringBuilder's reverse() method for easy string reversal [OK]

Common Mistakes:

• Trying to call reverse() directly on String
• Using non-existent String.reverse() method
• Assuming toCharArray() has reverse() method

2. Which of the following code snippets correctly reverses a string str using a for loop in Java?

easy

A. for (int i = 0; i < str.length(); i++) reversed += str.charAt(i);

B. for (int i = 1; i <= str.length(); i++) reversed += str.charAt(i);

C. for (int i = str.length() - 1; i >= 0; i--) reversed += str.charAt(i);

D. for (int i = str.length(); i > 0; i--) reversed += str.charAt(i);

Solution

1. Step 1: Understand string indexing in Java

   String indices start at 0 and go to length()-1.

2. Step 2: Identify correct loop to reverse string

   Loop from last index (length()-1) down to 0, appending characters to reversed string.

3. Final Answer:

   for (int i = str.length() - 1; i >= 0; i--) reversed += str.charAt(i); -> Option C

4. Quick Check:

   Loop from end to start = correct reversal [OK]

Hint: Loop backward from length()-1 to 0 to reverse string [OK]

Common Mistakes:

• Looping forward instead of backward
• Using 1-based indexing instead of 0-based
• Accessing charAt with index equal to length()

3. What is the output of the following Java code?

String str = "hello";
String reversed = "";
for (int i = str.length() - 1; i >= 0; i--) {
    reversed += str.charAt(i);
}
System.out.println(reversed);

medium

A. hello

B. olleh

C. olle

D. SyntaxError

Solution

1. Step 1: Trace the for loop from end to start

   The loop starts at index 4 ('o') and goes down to 0 ('h'), appending each character to reversed.

2. Step 2: Build reversed string step-by-step

   Characters appended in order: 'o', 'l', 'l', 'e', 'h' forming "olleh".

3. Final Answer:

   olleh -> Option B

4. Quick Check:

   Reverse of "hello" = "olleh" [OK]

Hint: Appending chars backward builds reversed string [OK]

Common Mistakes:

• Expecting original string output
• Confusing loop direction
• Assuming syntax error due to += on strings

4. Identify the error in the following Java code that attempts to reverse a string:

String str = "world";
String reversed = "";
for (int i = 0; i <= str.length(); i++) {
    reversed += str.charAt(i);
}
System.out.println(reversed);

medium

A. No error, code runs correctly

B. String reversed should be a StringBuilder instead of String

C. Use str.reverse() instead of loop

D. Loop condition should be i < str.length() to avoid IndexOutOfBoundsException

Solution

1. Step 1: Check loop boundary conditions

   The loop runs from i=0 to i=str.length() inclusive, but valid indices are 0 to str.length()-1.

2. Step 2: Identify exception cause

   Accessing str.charAt(str.length()) causes IndexOutOfBoundsException.

3. Final Answer:

   Loop condition should be i < str.length() to avoid IndexOutOfBoundsException -> Option D

4. Quick Check:

   Index must be less than length() [OK]

Hint: Use i < length() in loops to avoid index errors [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Ignoring exception from invalid index
• Assuming StringBuilder is mandatory

5. Given a string str, which approach efficiently reverses it without using extra space for another string in Java?

hard

A. Convert to char array, swap characters from ends moving inward, then create new string

B. Use StringBuilder.reverse() which creates a new reversed string internally

C. Concatenate characters in reverse order using a loop and a new string

D. Use recursion to build reversed string by concatenation

Solution

1. Step 1: Understand in-place reversal concept

   Strings in Java are immutable, so we cannot change them directly, but char arrays can be modified in place.

2. Step 2: Use char array swapping to reverse without extra string

   Convert string to char array, swap characters from start and end moving inward, then create string from modified array.

3. Step 3: Compare with other options

   Other methods create new strings or use extra space, so not in-place.

4. Final Answer:

   Convert to char array, swap characters from ends moving inward, then create new string -> Option A

5. Quick Check:

   In-place reversal uses char array swaps [OK]

Hint: Use char array swaps for in-place string reversal [OK]

Common Mistakes:

• Assuming StringBuilder.reverse() is in-place
• Using string concatenation causing extra space
• Ignoring string immutability in Java