Example: First Non-Repeating Character using Frequency Array in Java

// First Non-Repeating Character using Frequency Array
import java.util.Optional;

// Main class containing the entry point of the program
public class Main {
    public static void main(String[] args) {
        String input1 = "swiss";
        String input2 = "teeter";
        String input3 = "aabbcc";
        String input4 = "java";
        String input5 = "";
        String input6 = "abacaba"; // Example with repeating but first unique char

        System.out.println("Input: \"" + input1 + "\" -> First non-repeating character: " + findFirstNonRepeatingCharFrequencyArray(input1).orElse(' '));
        System.out.println("Input: \"" + input2 + "\" -> First non-repeating character: " + findFirstNonRepeatingCharFrequencyArray(input2).orElse(' '));
        System.out.println("Input: \"" + input3 + "\" -> First non-repeating character: " + findFirstNonRepeatingCharFrequencyArray(input3).orElse(' '));
        System.out.println("Input: \"" + input4 + "\" -> First non-repeating character: " + findFirstNonRepeatingCharFrequencyArray(input4).orElse(' '));
        System.out.println("Input: \"" + input5 + "\" -> First non-repeating character: " + findFirstNonRepeatingCharFrequencyArray(input5).orElse(' '));
        System.out.println("Input: \"" + input6 + "\" -> First non-repeating character: " + findFirstNonRepeatingCharFrequencyArray(input6).orElse(' '));
    }

    public static Optional<Character> findFirstNonRepeatingCharFrequencyArray(String str) {
        // Step 1: Handle null or empty string
        if (str == null || str.isEmpty()) {
            return Optional.empty();
        }

        // Step 2: Initialize a frequency array for ASCII characters (0-255)
        int[] charCounts = new int[256]; // Assuming ASCII or extended ASCII

        // Step 3: Populate the frequency array by iterating through the string once
        for (char c : str.toCharArray()) {
            // Ensure character is within array bounds, otherwise handle error or expand array
            if (c < 256) {
                charCounts[c]++;
            } else {
                // Handle non-ASCII characters if necessary, e.g., using a Map instead
                // For simplicity, we'll assume ASCII/extended ASCII for this approach
                System.err.println("Warning: Character out of ASCII range ignored: " + c);
            }
        }

        // Step 4: Iterate through the string again to find the first character with a count of 1
        for (char c : str.toCharArray()) {
            if (c < 256 && charCounts[c] == 1) {
                return Optional.of(c);
            }
        }

        // Step 5: If no non-repeating character is found
        return Optional.empty();
    }
}