Remove Duplicate Characters without HashMap

public class RemoveDuplicateCharacters {

public static void main(String[] args) {

// convert aabbbccccdddd to abcd

String  rs = "";

int j;

String str ="aabbbccccdddd" ;

str = str.toLowerCase();  //convert to lower case

char[] str1 = str.toCharArray();

for(int i =0;i< str1.length;i++) {

for( j= i+1; j<str1.length;j++) {

if (str1[i]== str1[j]) {

str1[j] = 'R';

}

}

}

for(char word: str1) {

if(word != 'R') {

rs = rs + word + " ";

}

// Display given String after removing duplicates

}

System.out.println("\nSentence after removing duplicate words:" + rs);

}

}