17. Letter Combinations of a Phone Number(bfs)

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.

class Solution {
    public List<String> letterCombinations(String digits) {
        LinkedList<String> res = new LinkedList<String>();
        if(digits.isEmpty()) return res;
        String[] mapping = new String[] {"0", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        res.add("");
        for(int i=0;i<digits.length();i++){
            int x=Character.getNumericValue(digits.charAt(i));
            while(res.peek().length()==i){
                String t = res.remove();
                for (char s :mapping[x].toCharArray())
                    res.add(t+s);
            }
        }
        return res;
    }
}