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.

class Solution {

private:

    vector<string> res;

    string temp;

public:

    void getRes(string digits,int len,map<char,string> num_map,int index)

    {

        if(index>=len)

        {

            res.push_back(temp);

            return;

        }

        char   strNum=digits[index];

        string strLetter=num_map[strNum];

        for(int j=;j<strLetter.size();j++)

        {

            temp.push_back(strLetter[j]);

            getRes(digits,len,num_map,index+);

            temp.pop_back();

        }

    }

    vector<string> letterCombinations(string digits) {

        map<char,string> num_map;

        num_map['']="";

        num_map['']="";

        num_map['']="abc";

        num_map['']="def";

        num_map['']="ghi";

        num_map['']="jkl";

        num_map['']="mno";

        num_map['']="pqrs";

        num_map['']="tuv";

        num_map['']="wxyz";

        int len=digits.size();

        if(len==)

            return res;

        getRes(digits,len,num_map,);

        return res;

    }

};

巴特西

Letter Combinations of a Phone Number——简单的回溯算法

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