[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/