Leetcode || Letter Combinations of a Phone Number

Given a digit string, return all possible letter combinations that the number could represent.

不知道有多少数字，很难遍历，那就用遍历的极端情况，递归

package pack;

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

class Solution {
public List<String> letterCombinations(String digits) {
HashMap<Integer, String> map = new HashMap<Integer, String>();
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");
map.put(0, "");

ArrayList<String> result = new ArrayList<String>();
ArrayList<Character> tmp = new ArrayList<Character>();

if(digits == null || digits.length() == 0)
return result;

getString(digits, tmp, result, map);

return result;

}
private void getString(String digits, ArrayList<Character> tmp, ArrayList<String> result, HashMap<Integer, String> map) {
if(digits.length() == 0) {    //递归到最后一个
char[] arr = new char[tmp.size()];  //将list类型tmp中数据变为字符串
for(int i=0; i<tmp.size(); i++){
arr[i] = tmp.get(i);
}
result.add(String.valueOf(arr));
return;
}

Integer curr = Integer.valueOf(digits.substring(0,1));
String s = (String)map.get(curr);
for(int i=0; i<s.length(); i++) {

tmp.add(s.charAt(i));
getString(digits.substring(1), tmp, result, map);
tmp.remove(tmp.size()-1);  //每次增加一个字符
}
}

}

public class Main {

public static void main(String[] args) {

System.out.println(new Solution().letterCombinations("2"));

}

}