Remove Duplicate Characters

/*
 * Problem : Process one line of text.
 * Input   : One line of text.
 * Output  : Unique characters in order.
 * Approach: Use a set.
 * Time    : O(n)
 * Space   : O(k)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
    // This method solves the task. It receives the input and returns the answer.
    static String solve(String s) {
        // Step 1: A set remembers the characters we have already seen.
        java.util.Set<Character> seen = new java.util.HashSet<>();
        StringBuilder result = new StringBuilder();

        for (char c : s.toCharArray()) {
            // Step 2: add returns true only when the character is new.
            if (seen.add(c)) {
                // Step 3: Keep the first occurrence.
                result.append(c);
            }
        }
        return result.toString();
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String s = scanner.hasNextLine() ? scanner.nextLine() : "";
        System.out.println(solve(s));
    }
}