String Reversal Approaches in DSA Java - Execution Trace

Concept Flow - String Reversal Approaches

Start with original string

↓

Choose reversal method

↓

Using loop

↓

Build reversed string step by step

↓

Return reversed string

↓

End

The flow shows starting with the original string, choosing a reversal method, building the reversed string, and returning it.

Execution Sample

DSA Java

String reverseLoop(String s) {
  String result = "";
  for (int i = s.length() - 1; i >= 0; i--) {
    result += s.charAt(i);
  }
  return result;
}

This code reverses a string by looping from the end to the start and building a new reversed string.

Execution Table

Step	Operation	Index i	Character Added	Result String
1	Initialize result	-	-	""
2	i = 4 (last index)	4	'o'	"o"
3	i = 3	3	'l'	"ol"
4	i = 2	2	'l'	"oll"
5	i = 1	1	'e'	"olle"
6	i = 0	0	'h'	"olleh"
7	Loop ends, return result	-	-	"olleh"

💡 Loop ends when i < 0 (i becomes -1), all characters added in reverse order.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	Final
i	-	4	3	2	1	0	-1 (loop ends)
result	""	"o"	"ol"	"oll"	"olle"	"olleh"	"olleh"

Key Moments - 3 Insights

Why does the loop start at s.length() - 1 and not 0?

Because string indexes start at 0, s.length() - 1 is the last character's index. We reverse by starting from the end (see execution_table steps 2-6).

Why do we add characters to result using += instead of inserting at the start?

Appending characters from the end to the result builds the reversed string naturally. Inserting at the start would be less efficient and more complex.

What happens when i becomes -1?

The loop condition i >= 0 fails, so the loop stops (see exit_note). This means all characters have been processed.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of result after step 4?

A "ol"

B "oll"

C "olle"

D "o"

💡 Hint

Check the 'Result String' column at step 4 in execution_table.

At which step does the loop add the character 'e' to the result?

A Step 5

B Step 2

C Step 3

D Step 6

💡 Hint

Look at the 'Character Added' column in execution_table for 'e'.

If the input string was empty (""), what would happen in the execution_table?

A The loop would throw an error

B The loop would run once adding a character

C The loop would not run and result would be ""

D The loop would run infinitely

💡 Hint

Consider the loop condition i = s.length() - 1; if s.length() is 0, i starts at -1.

Concept Snapshot

String Reversal Approaches:
- Loop from last index (length-1) to 0
- Append each character to result string
- Return result after loop ends
- Alternative: recursion or built-in reverse
- Loop stops when index < 0

Full Transcript

This concept shows how to reverse a string by looping from the last character to the first. We start with an empty result string. Each loop step adds one character from the original string's end to the result. When the loop index becomes less than zero, the loop stops and the reversed string is returned. This method is simple and builds the reversed string step-by-step.

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