1. 就是深搜。深搜三原則：終止條件，當(dāng)下操作，回退。

2. 寫完發(fā)現(xiàn)，這題早就做過了。

代碼如下：?

class?Solution?{

????public?List<String>?letterCombinations(String?digits)?{

????????if(null==digits?||?digits.equals("")){

????????????return?new?ArrayList<String>();

????????}

????????Map<Character,?String>?map?=?new?HashMap<Character,?String>();

????????//?map.put('1',)

????????map.put('2',?"abc");

????????map.put('3',?"def");

????????map.put('4',?"ghi");

????????map.put('5',?"jkl");

????????map.put('6',?"mno");

????????map.put('7',?"pqrs");

????????map.put('8',?"tuv");

????????map.put('9',?"wxyz");

????????List<String>?result?=?new?ArrayList<String>();

????????StringBuffer?sb?=?new?StringBuffer("");

????????letterCombinationsHelp(result,?map,?digits,?0,?sb);

????????return?result;

????}

????void?letterCombinationsHelp(List<String>?result,?Map<Character,?String>?map,?String?digits,int?index,?StringBuffer?sb??){

????????if(index?==?digits.length()?){

????????????result.add(sb.toString());

????????????return;

????????}

????????String?letterList?=?map.get(digits.charAt(index));

????????for(int?i?=?0;?i?<?letterList.length();?i++){

????????????sb.append(letterList.charAt(i));

????????????letterCombinationsHelp(result,?map,?digits,?index+1,?sb);

????????????sb.deleteCharAt(index);

????????}

????}

}