String Reversal Approaches in DSA Java - Time & Space Complexity

Time Complexity: String Reversal Approaches

O(n)

Understanding Time Complexity

We want to understand how long it takes to reverse a string as the string gets longer.

How does the work grow when the input string size increases?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

public String reverseString(String s) {
    char[] chars = s.toCharArray();
    int left = 0, right = chars.length - 1;
    while (left < right) {
        char temp = chars[left];
        chars[left] = chars[right];
        chars[right] = temp;
        left++;
        right--;
    }
    return new String(chars);
}

This code reverses a string by swapping characters from the start and end moving towards the center.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Swapping characters inside a while loop.
• How many times: About half the length of the string (n/2 times).

How Execution Grows With Input

As the string gets longer, the number of swaps grows roughly in half proportion to the string length.

Input Size (n)	Approx. Operations
10	5 swaps
100	50 swaps
1000	500 swaps

Pattern observation: The operations grow linearly with input size, just half as many swaps as characters.

Final Time Complexity

Time Complexity: O(n)

This means the time to reverse the string grows directly in proportion to the string length.

Common Mistake

[X] Wrong: "Reversing a string takes constant time because we just swap characters."

[OK] Correct: Each character must be visited and swapped, so the time grows with string length, not fixed.

Interview Connect

Understanding how string reversal scales helps you explain efficiency clearly in interviews and shows you can analyze simple loops well.

Self-Check

"What if we used recursion to reverse the string instead of a loop? How would the time complexity change?"

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