[leetcode]Letter Combinations of a Phone Number

题目描述如下：

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”].

虽然是medium难度的题目，但其实实现并不困难，一个递归便可以解决问题，需要一个辅助函数来生成最终的list。代码如下：

public class Solution {    List<String> resList = new ArrayList<>();    List<String> subList = new ArrayList<>();    public List<String> letterCombinations(String digits) {        if(digits.length() == 0) return resList;        String[] arr={"", "" ,"abc", "def", "ghi", "jkl","mno", "pqrs", "tuv", "wxyz" };        for(int i = 0; i < digits.length(); i++){            String s = arr[(int)( digits.charAt(i) - '0')];            subList.add(s);        }        getResultList( "", 0, digits.length());        return resList;    }    public void getResultList(String tmpStr, int step, int digitsLen){        if(tmpStr.length() == digitsLen) resList.add(tmpStr);        else{            String s = subList.get(step);            for(int i = 0; i < s.length(); i++){                tmpStr += s.charAt(i);                getResultList(tmpStr, step + 1, digitsLen);                tmpStr = tmpStr.substring(0, tmpStr.length() - 1);            }        }    }}

题目链接：https://leetcode.com/problems/letter-combinations-of-a-phone-number/