17. Letter Combinations of a Phone Number

class Solution {
    char a[][]={{'a','b','c'},{'d','e','f'},{'g','h','i'},{'j','k','l'},{'m','n','o'},{'p','q','r','s'},{'t','u','v'},{'w','x','y','z'}};
    public void backtrack(List<String> s, String digits, String word, int i)
    {
        if(i == digits.length())
        {
            s.add(word);
            return;
        }
        for(int j=0; j<a[digits.charAt(i)-50].length;j++)
        {
            backtrack(s, digits, word + a[digits.charAt(i)-50][j], i+1);
        }
    }
    public List<String> letterCombinations(String digits) 
    {
        List<String> s = new ArrayList<>();
        if(digits.length() == 0)
            return s;
        backtrack(s, digits, "", 0);
        return s;
        
    }
}