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给定一个仅包含数字 2-9 的字符串(类似目前功能机上的键)，返回所有它能表示的字母组合。

            2---abc
            3---def
            4---ghi
            5---jkl
            6---mno
            7---pqrs
            8---tuv
            9---wxyz

给出数字到字母的映射如下（与电话按键相同）。注意 1 不对应任何字母。
示例:

输入："23"
输出：["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

package com.loo;

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

public class PhoneNumberChar {
    public static void main(String[] args) {
        String str = "239";
        List<String> list = getPhoneNumberChar(str);
        for (String s : list) {
            System.out.println(s);
        }
    }

    public static List<String> getPhoneNumberChar(String digits) {
        List<String> coms = new ArrayList<String>();
        if (digits == null || "".equals(digits) || digits.length()==0) {
            return coms;
        }
        Map<Character , String> phoneNumMap = new HashMap<Character , String>();
        phoneNumMap.put('2' , "abc");
        phoneNumMap.put('3' , "def");
        phoneNumMap.put('4' , "ghi");
        phoneNumMap.put('5' , "jkl");
        phoneNumMap.put('6' , "mno");
        phoneNumMap.put('7' , "pqrs");
        phoneNumMap.put('8' , "tuv");
        phoneNumMap.put('9' , "wxyz");
        combinationNum(coms , phoneNumMap , digits , 0 , new StringBuffer());
        return coms;
    }

    public static void combinationNum(List<String> coms , Map<Character , String> phoneNumMap , String digits , int index , StringBuffer combination) {
        if (index == digits.length()) {
            coms.add(combination.toString());
        } else {
            char digit = digits.charAt(index);
            String phoneNumString = phoneNumMap.get(digit);
            int length = phoneNumString.length();
            for (int i=0;i<length;i++) {
                combination.append(phoneNumString.charAt(i));
                combinationNum(coms , phoneNumMap , digits , index+1 , combination);
                combination.deleteCharAt(index);
            }
        }
    }
}