LeetCode-电话号码的字母组合

给定一个仅包含数字 2-9 的字符串，返回所有它能表示的字母组合。

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"].

说明:

尽管上面的答案是按字典序排列的，但是你可以任意选择答案输出的顺序。

class Solution {
    public List<String> letterCombinations(String digits) {
 
    	ArrayList<String> list = new ArrayList<>();
    	
    	if(digits==null||digits.length()==0)
    	{
    		return list;
    	}
    	
    	String[] phoneNumbers = {" ", " ", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
    	dfsPhoneNumbers(digits,0,phoneNumbers,list,new StringBuffer());
    	
    	return list;
    }

	private void dfsPhoneNumbers(String digits,int index, String[] phoneNumbers, ArrayList<String> list,StringBuffer buffer) 
	{
		// TODO Auto-generated method stub
		if(index==digits.length())
		{
			list.add(buffer.toString());
			return;
		}
		String phoneNumberString = phoneNumbers[digits.charAt(index)-'0'];
		
		for(int i=0;i<phoneNumberString.length();++i)
		{
			buffer.append(phoneNumberString.charAt(i));
			dfsPhoneNumbers(digits, index+1, phoneNumbers, list, buffer);
			buffer.deleteCharAt(buffer.length()-1);
		}
	    
		
	}
}