Mirror Characters of a String in Java

Mirroring characters of a string replaces each letter with its counterpart from the opposite end of the alphabet (e.g., 'a' becomes 'z', 'b' becomes 'y'). Uppercase and lowercase letters retain their case, and non-alphabet characters remain unchanged. This technique is useful in encoding and text transformations.

Example 1:

Input:

N = 2

hello

Output:

heovv

(Mirroring starts from the 2nd position)

Example 2:

Input:

N = 4

programming

Output:

progmlmtorn

(Mirroring starts from the 4th position)

Example 3:

Input:

N = 5

fantastic

Output:

fantzghxr

(Mirroring starts from the 5th position)

Approach: Mathematical Character Mapping

Mathematical Character Mapping involves transforming characters by calculating their mirrored counterparts based on their positions in the alphabet using arithmetic operations. This approach efficiently replaces each character with its opposite by leveraging the ASCII values, allowing for a straightforward implementation without the need for additional data structures.

Algorithm

Step 1: A string input containing a mix of uppercase letters, lowercase letters, and non-alphabet characters.

Step 2: Create a StringBuilder called mirrored to build the mirrored string efficiently.

Step 3: For each character ch: a. Check if ch is a lowercase letter:

• If ch is between 'a' and 'z', calculate the mirrored character using:
  
  • mirroredChar = (char) ('z' - (ch - 'a')) b. Check if ch is an uppercase letter:
• If ch is between 'A' and 'Z', calculate the mirrored character using:
  
  • mirroredChar = (char) ('Z' - (ch - 'A')) c. If ch is not a letter:
• Append it as-is to mirrored (no transformation is needed).

Step 4: After determining the mirrored character (or the unchanged non-alphabet character), append it to the mirrored string.

Step 5: Once all characters are processed, convert the StringBuilder to a string and return it.

Implementation

File Name: MirrorCharacters.java

Output:

 
zyxCBA   

Time Complexity: O(n), where n is the length of the input string.

Auxiliary Space Complexity: O(n) for storing the mirrored result.

Approach: Using Mapping

A HashMap is used to assign each alphabet character to its corresponding mirrored character (e.g., 'a' to 'z', 'b' to 'y'). The program creates a new mirrored string by efficiently manipulating the input string using a StringBuilder to find and replace each character with its mirror in the map.

Algorithm

Step 1: Start with an input string (e.g., "abcXYZ").

Step 2: Call the mirrorString() Function with the input string to generate its mirrored version.

Step 3: In the mirrorString() function, first call the createMirrorMap() to create a map that defines the mirror relationships for characters.

Step 4: Initialize a HashMap<Character, Character> to store mappings of each character to its mirrored counterpart.

Step 5: In the mirrorString() function, use a StringBuilder to build the mirrored version of the input string.

• For each character ch in the input string:
  
  • If the character exists in the mirror map:
    
    • Append the mirrored character (from the map) to the StringBuilder.
  • If the character does not exist in the mirror map (e.g., non-alphabet characters), append the character as it is.
• Convert the StringBuilder to a string and return the mirrored string.

Implementation

File Name: MirrorCharactersUsingMap.java

Output:

 
zyxCBA   

Time Complexity: O(n), where n is the length of the input string.

Auxiliary Space Complexity: O(n) for storing the mirrored string.