Letter Combinations of a Phone Number

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.

思路：查找电话号码对应的英语单词组合，可以参考《编程之美》3.2电话号码对应英语单词。主要使用递归函数的调用，找出电话号码对应的所有英语单词的组合。

class Solution {

public:

    void RecursiveSearch(vector<string> &result,string &path,const string &digits,int index)

    {

        string str[10]={"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};

        if(index==digits.size())

        {

            result.push_back(path);

            return;

        }

        for(int i=0;i<str[digits[index]-'0'].size();i++)

        {

            path.push_back(str[digits[index]-'0'][i]);

            RecursiveSearch(result,path,digits,index+1);

            path.pop_back();

        }

    }

    vector<string> letterCombinations(string digits) {

        vector<string> result;

        result.clear();

        string path;

        path.clear();

        RecursiveSearch(result,path,digits,0);

        return result;

    }

};