Letter Combinations of a Phone Number

题目原型：

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.

基本思路：

递归求解

	public ArrayList<String> letterCombinations(String digits)
	{
		ArrayList<String> list = new ArrayList<String>();
		char[] number = digits.toCharArray();//存放电话号码
		int len = number.length;//电话号码长度
		String[] phone = {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};//字母与数字对应表
		int[] total = {0,0,3,3,3,3,3,4,3,4};//每个数字上有多少个字母
		int[] answer = new int[len];//指示每个数字所对应字母序列的字符位子,如第一个数字是2，如果answer[0] = 0,表示所对应的字符是a即phone[0].charAt[answer[0]]=‘a‘
		combinatons(list,phone, number, answer, 0, len, total);
		return list;
	}
	
	void combinatons(ArrayList<String> list,String[] phone,char[] number , int[] answer , int index , int len , int[] total)
	{
		if(index==len)
		{
			StringBuffer strbuf = new StringBuffer();
			for(int i = 0 ;i<len;i++)
			{
				strbuf.append(phone[number[i]-‘0‘].charAt(answer[i]));
			}
			list.add(strbuf.toString());
			return;
		}
		for(answer[index]=0;answer[index]<total[number[index]-‘0‘];answer[index]++)
		{
			combinatons(list,phone, number, answer, index+1, len, total);
		}
	}

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Letter Combinations of a Phone Number

原文：http://blog.csdn.net/cow__sky/article/details/21184979