LeetCode: Letter Combinations of a Phone Number 解题报告

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.

SOLUTION 1:

经典递归模板。

注意这一行不要写错了，根据当前的数字来取得可能的字母组合：

//get the possiable selections.
String s = map[digits.charAt(index) - '0'];

 1 public class Solution {

 2     String[] map = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};

 3     

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

 5         List<String> ret = new ArrayList<String>();

 6         if (digits == null) {

 7             return ret;

 8         }

 9         

10         dfs(digits, new StringBuilder(), ret, 0);

11         return ret;

12     }

13     

14     public void dfs(String digits, StringBuilder sb, List<String> ret, int index) {

15         int len = digits.length();

16         if (index == len) {

17             ret.add(sb.toString());

18             return;

19         }

20         

21         // get the possiable selections.

22         String s = map[digits.charAt(index) - '0'];

23         for (int i = 0; i < s.length(); i++) {

24             sb.append(s.charAt(i));

25             dfs(digits, sb, ret, index + 1);

26             sb.deleteCharAt(sb.length() - 1);

27         }

28     }

29     

30 }

View Code

GITHUB:

https://github.com/yuzhangcmu/LeetCode_algorithm/blob/master/combination/LetterCombinations.java