String Reversal Approaches in DSA Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to reverse a string using a for loop.

DSA Java

String reversed = "";
for (int i = str.length() - 1; i [1] 0; i--) {
    reversed += str.charAt(i);
}

Drag options to blanks, or click blank then click option'

A>

B<=

C<

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>=' causes the first character to be skipped.

Using '>' causes the loop to stop before reaching index 0.

The loop should continue while i is greater than or equal to 0 to include the first character at index 0.

2fill in blank

medium

Complete the code to reverse a string using StringBuilder's built-in method.

DSA Java

StringBuilder sb = new StringBuilder(str);
sb.[1]();
String reversed = sb.toString();

Drag options to blanks, or click blank then click option'

AtoString

Breverse

Cappend

Ddelete

Attempts:

3 left

💡 Hint

Common Mistakes

Using toString() alone does not reverse the string.

Using append() or delete() does not reverse the string.

The reverse() method of StringBuilder reverses the characters in the builder.

3fill in blank

hard

Fix the error in the recursive string reversal method by completing the base case condition.

DSA Java

public String reverse(String str) {
    if (str.length() [1] 0) {
        return "";
    }
    return reverse(str.substring(1)) + str.charAt(0);
}

Drag options to blanks, or click blank then click option'

A<

B>

C==

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' or '>=' causes infinite recursion.

Using '<' does not correctly identify the base case.

The base case should check if the string length is zero to stop recursion.

4fill in blank

hard

Fill both blanks to reverse a string using a character array and swapping.

DSA Java

char[] chars = str.toCharArray();
int left = 0;
int right = str.length() [1] 1;
while (left [2] right) {
    char temp = chars[left];
    chars[left] = chars[right];
    chars[right] = temp;
    left++;
    right--;
}
String reversed = new String(chars);

Drag options to blanks, or click blank then click option'

A-

B<

C>

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Setting right to length instead of length - 1 causes index error.

Using '>' instead of '<' in while condition causes no swaps.

right should start at length - 1 (last index), and loop continues while left < right to swap characters.

5fill in blank

hard

Fill all three blanks to reverse a string using a for loop and StringBuilder.

DSA Java

StringBuilder sb = new StringBuilder();
for (int i = str.length() [1] 1; i [2]= 0; i[3]) {
    sb.append(str.charAt(i));
}
String reversed = sb.toString();

Drag options to blanks, or click blank then click option'

A-

B>

C--

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' instead of '--' causes infinite loop.

Incorrect loop condition skips first character.

Start from length - 1, loop while i >= 0, and decrement i with -- to append characters in reverse order.

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