Example: First Non-Repeating Character (HashMap) in Java

// First Non-Repeating Character (HashMap)
import java.util.HashMap;
import java.util.Map;
import java.util.Scanner; // Included as per code pattern, though not strictly needed for the core logic of this problem.

public class Main {
    public static void main(String[] args) {
        String s1 = "leetcode";
        System.out.println("String: \"" + s1 + "\", First non-repeating char index: " + firstUniqCharHashMap(s1));

        String s2 = "loveleetcode";
        System.out.println("String: \"" + s2 + "\", First non-repeating char index: " + firstUniqCharHashMap(s2));

        String s3 = "aabb";
        System.out.println("String: \"" + s3 + "\", First non-repeating char index: " + firstUniqCharHashMap(s3));
    }

    public static int firstUniqCharHashMap(String s) {
        // Step 1: Create a HashMap to store character frequencies.
        // Key: character, Value: count of occurrences
        Map<Character, Integer> charCounts = new HashMap<>();

        // Step 2: Iterate through the string to populate the frequency map.
        // For each character, increment its count.
        for (char c : s.toCharArray()) {
            charCounts.put(c, charCounts.getOrDefault(c, 0) + 1);
        }

        // Step 3: Iterate through the string again to find the first character with a count of 1.
        // The order of iteration is crucial here to find the *first* non-repeating character.
        for (int i = 0; i < s.length(); i++) {
            if (charCounts.get(s.charAt(i)) == 1) {
                return i; // Found the first non-repeating character
            }
        }

        // Step 4: If no non-repeating character is found after checking all characters, return -1.
        return -1;
    }
}