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.
解法是递推，下一次的list就是当前list的string加上当前数字代表的字母。
1 public class Solution { 2 List<String> lastList=new ArrayList<>(); 3 String[] N2=new String[]{"a","b","c"}; 4 String[] N3=new String[]{"d","e","f"}; 5 String[] N4=new String[]{"g","h","i"}; 6 String[] N5=new String[]{"j","k","l"}; 7 String[] N6=new String[]{"m","n","o"}; 8 String[] N7=new String[]{"p","q","r","s"}; 9 String[] N8=new String[]{"t","u","v"}; 10 String[] N9=new String[]{"w","x","y","z"}; 11 12 public List<String> letterCombinations(String digits) { 13 String[] num = null; 14 if (digits.equals("")) { 15 lastList.add(""); 16 return lastList; 17 } 18 for (int i = 0; i < digits.length(); i++) { 19 20 switch (digits.charAt(i)) { 21 case '2': 22 num=N2; 23 break; 24 case '3': 25 num=N3; 26 break; 27 case '4': 28 num=N4; 29 break; 30 case '5': 31 num=N5; 32 break; 33 case '6': 34 num=N6; 35 break; 36 case '7': 37 num=N7; 38 break; 39 case '8': 40 num=N8; 41 break; 42 case '9': 43 num=N9; 44 break; 45 46 } 47 if (i==0) { 48 lastList.addAll(Arrays.asList(num)); 49 continue; 50 } 51 List<String> nextList=new ArrayList<>(); 52 for (String string : lastList) { 53 for (String ss : num) { 54 nextList.add(string+ss); 55 } 56 } 57 lastList=nextList; 58 } 59 return lastList; 60 } 61 62 }