Program to find k-th Lexicographical String from all Happy Strings of length n in Java

A happy string is a string that:

• consists only of letters of the set ['a', 'b', 'c'].
• s[i] != s[i + 1] for all values of i from 1 to s.length - 1 (string is 1-indexed).

class MainProgram{
    private String getHappyString(int n, int k) {
        char[] arr = {'a', 'b', 'c'};
        String res="";
        List<String> list=new ArrayList<>();
        generatePerm(arr, n, res, list);
        if(list.size() >= k)
            res=list.get(k-1);
        return res;
    }
    
    private void generatePerm(char[] arr, int n, String res,  
        List<String> list){
        if(n == 0){
            list.add(res);
            return;
        }
        
        for(int i=0; i<arr.length; i++){
           if(res == "" || res.charAt(res.length()-1) != arr[i]){
                String pre=res+arr[i];
                generatePerm(arr, n-1, pre, list);
            }
        }   
    }
    public static void main(String args[]){
        System.out.println("Happy String: "+getHappyString(3,4));
    }
}

Output: Happy String: acb

Hope it helps, enjoy coding!

Any improvement or suggestions are welcomed !