Letter Combinations of a Phone Number

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本文介绍了一种基于电话按键映射的数字转字母组合算法，通过递归调用实现字符串到对应字母的转换，输出所有可能的组合。

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.

public class Solution {
    public ArrayList<String> letterCombinations(String digits) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        ArrayList<String> res = new ArrayList<String>();  
        get_comb(res, digits, new StringBuilder(""), 0);  
        return res; 
    }
    public void get_comb(ArrayList<String> res, String digits, StringBuilder s, int pos){  
        String[] letters = {"0", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};  
        if(pos == digits.length()){  
            res.add(s.toString());  
            return;  
        }  
        int num = digits.charAt(pos) - '0';  
        for(int i = 0; i < letters[num].length(); i++){  
            s.append(letters[num].charAt(i));  
            get_comb(res, digits, s, pos + 1);
            s.deleteCharAt(s.length() - 1);
        }  
    }  
}