leetcode 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.

使用队列保存中间的字符串，用迭代方法完成

public class Solution {
    public List<String> letterCombinations(String digits) {
    	LinkedList<String> ans = new LinkedList<String>();
    	String[] mapping = new String[] {"0", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        if(digits.length()==0) return ans;
    	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.poll();
            	for(char s : mapping[x].toCharArray())
                	ans.add(t+s);
        	}
    	}
    	return ans;
	}
}