Solution17.java

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class Solution17 {
 public List<String> letterCombinations(String digits) {
 if (digits==null||digits.length()==0){
 return new ArrayList<>();}
 Map<Character,String> map = new HashMap<>();
 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 res = new ArrayList<>();
 getCombinations("",digits,0,res,map);
 return res;
 }
 void getCombinations(String str,String digits,int position,List<String> res,Map<Character,String> map){
 if (digits.length()==position){
 res.add(str);
 return;
 }

 String letters = map.get(digits.charAt(position));

 for (int j = 0;j<letters.length();j++){
 //char和char不能直接相加，char+string可以，char+char返回的是int，ASCII对应整数
 getCombinations(str+letters.charAt(j),digits,position+1,res,map);
 }

 }
 public static void main(String[] args) {
 Solution17 s = new Solution17();
 System.out.println(s.letterCombinations("23"));


 }
}