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;
    }
};