如何不使用StringBuilder从字符串（而不是数组）中删除重复项？

Question

I am working with my program and I need to remove repeated characters from a string given to me by the user. I referenced other questions, but they are all using StringBuilder to remove duplicates. However, is there any way to remove duplicates without turning the string into an array, using StringBuilder and Set ?

I haven't learnt those yet, so I don't understand them very well. Could I get some help?

For example, if user types in happyrolling the result should be hapyroling .

Answer 1

It seems from your example you want to remove repeated characters (not words ).

You can use regex to find the repeats and remove them:

str = str.replaceAll("(.)\\1+", "$1");

This regex captures every character but only matches when followed by the same character by using a back reference to the captured group. The replacement is the captured character, so for example "xx" is replaced by "x"

Answer 2

Somehow you need to convert the string into character array. String is immutable in Java, so if you want to do any type of operation in the String, either you have to convert it to charArray or you have to use StringBuilder to create new String. You can keep track of the used character using hashmap.

public String removeDuplicates(String str){
    char[] array = str.toCharArray();
    char ch;
    int k = 0;
    HashMap<Character, Integer> hMap = new HashMap();
    for(int i = 0; i < str.length(); i++){
        ch = array[i];
        if(hMap.get(ch) == null){
            array[k++] = ch;
            hMap.put(ch, 1);
        }
    }
    return new String(array, 0, k);
}