17. Letter Combinations of a Phone Number

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.

题目含义：给定一个数字字符串（数字为0-9），请按手机的9宫格输入法得到所有可能的字母组合（上面举了个列子）。

    public List<String> letterCombinations(String digits) {
        if (digits.isEmpty()) return new ArrayList<>();
        LinkedList<String> ans = new LinkedList<String>();
        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)); //效率不如下面的高
            int x = digits.charAt(i) - '0';
            int size = ans.size();
//            BFS理论，把上一层的string拿出来，加入新的字母后重新加入
            for(int j=0;j<size;j++) 
            {
                String t = ans.remove();
                for (char s : mapping[x].toCharArray())
                    ans.add(t + s);
            }
        }
        return ans;
    }