LeetCode-Letter Combinations of a Phone Number

Description：
Given a string containing digits from 2-9 inclusive, 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. Note that 1 does not map to any letters.

Example:

Input: “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.

题意：给定一串数字，每个数字对应9键键盘上的字母，要求输出这些数字所对应的那些字母的所有排列组合；

解法：可以利用FIFO来实现所有的排列组合；我们首先将第一个数字对应的字母出入到队列中，在下轮，每次弹出一个字符串时，与后一个数字所对应的所有字母分别进行组合后入队列，这样，前两个数字的所有组合就在队列中了，重复这个操作，直到遍历完数字字符串的所有数字；

class Solution
    public List<String> letterCombinations(String digits) {
        LinkedList<String> result = new LinkedList<>();
        if(digits.length() == 0){
            return result;
        }//数字串为空
        String[] numToLetter = new String[] {"abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};//数字与字母映射表
        result.add("");
        for(int i=0; i<digits.length(); i++){
            int index = digits.charAt(i) - '0';//字符串中的数字
            while(result.peek().length() == i){
                String firstMatch =  result.remove();
                for(char x : numToLetter[index-2].toCharArray()){
                    result.add(firstMatch + x);
                }
            }
        }
        return