LCP.java

class Solution {
    public List<String> letterCombinations(String digits) {
        String[] set = new String[10];
        set[2] = "abc";
        set[3] = "def";
        set[4] = "ghi";
        set[5] = "jkl";
        set[6] = "mno";
        set[7] = "pqrs";
        set[8] = "tuv";
        set[9] = "wxyz";

        List<String> ans = new ArrayList<>();
        if(digits == null || digits.length()==0){
            return ans;
        }

        helper(ans, digits, set, 0, "");

        return ans;
    }

    private void helper(List<String> ans, String digits, String[] set, int index, String currString){
        if(index == digits.length()){
            ans.add(currString);
            return ;
        }

        String possibleChars = set[digits.charAt(index) - '0'];
        for(char c: possibleChars.toCharArray()){
            helper(ans, digits, set, index+1, currString + c);
        }

    }
}