A algorithm questions I couldn't solve

beauty.java

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.Scanner;


public class beauty {

  public static String solve(String input) {
    
    int n = input.length();
    ArrayList<Character> r = new ArrayList<Character>();
    char prevc = 'a';
    for (int i = 0; i < n; i++) {
      char c = input.charAt(i);
      if (!r.contains(c)) {
        r.add(c);
        prevc = c;
      }else { 
        // Determine if we keep this char or not
        int lastIndex = r.indexOf(c);
        if (lastIndex + 1 < r.size() && c < r.get(lastIndex+1)) {
          // Remove previous one
          r.remove(lastIndex);
          r.add(c);
          prevc = c;
        }
      }
    }
    
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < r.size(); i++) {
      sb.append(r.get(i));
    }
    return sb.toString();
  }
  
  public static void main(String[] args) {
    Scanner c = null;
    try {
      c = new Scanner(System.in);
    }catch(Exception e) {
      System.out.println(e.getMessage());
    }
    
    String str = c.nextLine();
    System.out.println(solve(str));
  }
}