LetterCombinationsofaPhoneNumber17.java

public class Solution {
    private ArrayList<String> res = new ArrayList<String>();
    
    public List<String> letterCombinations(String digits) {

        int length = digits.length();
        if(length==0) return res;
        String[] str = {"", "", "abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
        helper(0, digits, str, "");
        
        return res;
    }
    
    public void helper(int index, String digits, String[] str, String curr){
        if(index==digits.length()){
            res.add(curr);
            return;
        }
        String s1 = str[Integer.parseInt(""+digits.charAt(index))];
        for(int i=0;i<s1.length();i++){
            char ca = s1.charAt(i);
            curr=curr+ca;
            helper(index+1,digits,str,curr);
            curr=curr.substring(0, curr.length()-1);
        }
    }
}