[LeetCode]Letter Combinations of a Phone Number

题目要求如下：

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.

因为题目没有要求字母的排列顺序，所以处理起来相对容易很多。

题比较简单，用递归的方式就可以完成，因此就不做过多的思路介绍和代码注释了，看代码就可以明白。

代码如下，欢迎指导交流~

AC, Runtime: 8 ms

//LeetCode_Letter Combinations of Phone Number
//Written by zhou
//2013.11.27

class Solution {
public:

    void MapLetters(const string *map, string &digits, string &str, int curLen, vector &res)
    {
        if (curLen == digits.length())  //完成一个组合
        {
            res.push_back(str);
			return;
        }
        
		for (int i = 0; i < map[digits[curLen]-'0'].length(); ++i)  
        {
            str.push_back(map[digits[curLen]-'0'][i]);
            MapLetters(map,digits,str,curLen+1,res);
            str.pop_back();   
        }
    }
    
    vector letterCombinations(string digits) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        
        const string map[10] = {"","","abc","def","ghi","jkl","mno","qprs","tuv","wxyz"};
        vector result;
        string str("");   //临时字串
        MapLetters(map,digits,str,0,result); 
        return result;
    }
};