【leetcode】Letter Combinations of a Phone Number

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.

回溯法递归调用即可：

class Solution {

public:

    vector<string> letterCombinations(string digits) {

        vector<string> num2string(10);

        num2string[0]="";

        num2string[1] = "";  

        num2string[2] = "abc";  

        num2string[3] = "def";  

        num2string[4] = "ghi";  

        num2string[5] = "jkl";  

        num2string[6] = "mno";  

        num2string[7] = "pqrs";  

        num2string[8] = "tuv";  

        num2string[9] = "wxyz";

       

        vector<string> result;

        string tmp="";

        combination(digits,0,tmp,num2string,result);

        return result;

    }

   

   

    void combination(string &digits,int index,string tmp,vector<string> &num2string,vector<string> &result)

    {

        if(index==digits.length())

        {

            result.push_back(tmp);

            return;

        }

       

        string mapString=num2string[digits[index]-'0'];

        for(int i=0;i<mapString.length();i++)

        {

            tmp.push_back(mapString[i]);

            combination(digits,index+1,tmp,num2string,result);

            tmp.pop_back();

        }

    }

};