Letter Combinations of a Phone Number

17. Letter Combinations of a Phone Number

Difficulty: Medium

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.

My Solution

直接用一个递归解决

class Solution {
    String digits;
    List<String> l = new ArrayList<>();
    private String[] letter = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
    public List<String> letterCombinations(String digits) {
        this.digits = digits;
        get("", 0);
        
        return l;
    }
    
    public void get(String temp, int current){
        if(current>this.digits.length()-1){
            if(temp.length()>0)l.add(temp); // 记得判断空的输入
            return;
        }
        
        String value = letter[digits.charAt(current)-'0'];
        for(int i=0;i<value.length();++i){
            get(temp+value.charAt(i), current+1);
        }
    }
}