Letter Combinations of a Phone N

Given a digit string, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below.

Input:Digit string "23"
Output:["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.

大意就是返回给出数字的对应手机键盘上的所有可能的组合

  public List<String> letterCombinations(String digits) {
        LinkedList<String> result = new LinkedList<String>();
        if(digits == null || digits.length() == 0){
            return result;
        }

        String[] buttons = new String[]{"0", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        result.add("");
        /**
         * 利用FIFO
         */
        for(int i = 0; i < digits.length(); i++){
            int c = Character.getNumericValue(digits.charAt(i));


            while (result.peek().length() == i){
                /**
                 * 顶端弹出栈
                 */
                String p = result.remove();
                for(char cr : buttons[c].toCharArray()){
                    /**
                     * 入栈，在队列尾端增加
                     */
                    result.offer(p + cr);
                }
            }
        }
        return result;
    }