[leetcode] 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.

思路：回溯法DFS，思路很简单。

public class Solution {
public void dfs(String digits, ArrayList<String> result, char[] path, int index, String[] str){
if(index == str.length){
result.add(new String(path));
return;
}
for(int i = 0; i < str[index].length(); i++){
path[index] = str[index].charAt(i);
dfs(digits, result, path, index+1, str);
}
}
public ArrayList<String> letterCombinations(String digits) {
HashMap<Character, String> map = new HashMap<Character, String>();
map.put('0', "");
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");
ArrayList<String> result = new ArrayList<String>();
if(digits == null || digits.length() == 0)
return result;
String[] str = new String[digits.length()];
for(int i = 0 ; i < digits.length(); i++){
str[i] = map.get(digits.charAt(i));
}
char[] path = new char[digits.length()];
dfs(digits, result, path, 0, str);
return result;
}
}