[Leetcode]017. Letter Combinations of a Phone Number

    public List<String> letterCombinations(String digits) {

    LinkedList<String> ans = new LinkedList<String>();

    if(digits == null || digits.length() == 0) return ans;

    String[] mapping = new String[] {"0", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};

    ans.add("");

    for(int i =0; i<digits.length();i++){

        int x = Character.getNumericValue(digits.charAt(i));

        while(ans.peek().length()==i){

            String t = ans.remove();

            for(char s : mapping[x].toCharArray())

                ans.add(t+s);

        }

    }

    return ans;

    }