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.
算法思路：
1. 很明显，这道题是组合问题combination，可以使用迭代来完成
2. DFS。我的旧博客用的是combination的方法，不过当时心情不好，写的比较乱，这次我用的dfs，代码更简洁
1 public class Solution { 2 char[][] numberBoard = {{},{},{'a','b','c'},{'d','e','f'},{'g','h','i'},{'j','k','l'},{'m','n','o'},{'p','q','r','s'},{'t','u','v'},{'w','x','y','z'}}; 3 public List<String> letterCombinations(String digits) { 4 List<String> result = new ArrayList<String>(); 5 dfs(result,new StringBuilder(),digits); 6 return result; 7 } 8 9 private void dfs(List<String> result,StringBuilder sb,String suffix){ 10 if(suffix.length() == 0){ 11 result.add(sb.toString()); 12 return; 13 } 14 int index = suffix.charAt(0) - '0'; 15 for(int i = 0; i < numberBoard[index].length; i++){ 16 dfs(result, sb.append(numberBoard[index][i]), suffix.substring(1)); 17 sb.deleteCharAt(sb.length() - 1); 18 } 19 } 20 }
第二遍：
对上一遍代码进行了一点点修复，如果digits中包含0或者1时，应该做出相应处理。
代码如下：
1 public class Solution { 2 List<String> res = new ArrayList<String>(); 3 public List<String> letterCombinations(String digits) { 4 dfs(new StringBuilder(),0,digits); 5 return res; 6 } 7 private void dfs(StringBuilder sb,int k,String s){ 8 if(k >= s.length()){ 9 res.add(sb.toString()); 10 return; 11 } 12 char[][] board = {{},{},{'a','b','c'},{'d','e','f'},{'g','h','i'},{'j','k','l'},{'m','n','o'},{'p','q','r','s'},{'t','u','v'},{'w','x','y','z'}}; 13 int n = s.charAt(k) - '0'; 14 if(n < 2) dfs(sb,k,s);//跳过，也可以抛出异常,总之需要对非法数字进行处理 15 for(int i = 0; i < board[n].length; i++){ 16 sb.append(board[n][i]); 17 dfs(sb,k + 1,s); 18 sb.deleteCharAt(sb.length() - 1); 19 } 20 } 21 }