Letter Combinations of a Phone Number

https://leetcode.com/problems/letter-combinations-of-a-phone-number/

这是我一开始的思路，穷举而已。

String[] myMap=new String[]{"abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};    public List<String> letterCombinations(String digits) {        List<String> result=new ArrayList<String>();        int pointer=(int)Math.pow(4,digits.length());        if(pointer==1){return result;}        for(int i=0;i<pointer;i++){            String tmp="";            for(int j=0;j<digits.length();j++){                int index=3&(i>>j*2);                if(index>=myMap[digits.charAt(j)-'2'].length()){                    tmp=null;                    break;                }                tmp+=myMap[digits.charAt(j)-'2'].charAt(index);            }            if(tmp!=null){                result.add(tmp);            }        }        return result;    }

看到了一个别人用LinkedList完成的，觉得非常棒：

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"};    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.remove();            for(char s : mapping[x].toCharArray())                ans.add(t+s);        }    }    return ans;}