1. Letter Combinations of a Phone Number My Submissions QuestionEditorial Solution
   
   Total Accepted: 78554 Total Submissions: 273286 Difficulty: Medium
   
   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”].

Submission Details

25 / 25 test cases passed.

Status: Accepted

Runtime: 0 ms

思路：依次迭代，比较简单

class Solution {

public:

    vector<string> letterCombinations(string digits) {

        int n = digits.size();

        vector<string> res;

        map<char,string> nmap;

        init_map(nmap);

        for(int i=0;i<n;++i){

            if(digits[i]=='1')continue;//1代表空，无效数字越过

            string word= nmap[digits[i]];

            vector<string> sw,stmp;

            for(int j=0;j<word.size();++j){

                vector<char> vs;

                vs.push_back(word[j]);

                vs.push_back('\0');         //特别注意char* 转string

                sw.push_back(vs.data());

                for(int k=0;k<res.size();++k)

                    stmp.push_back(res[k]+sw[j]);

            }

            if(res.size()==0)res = sw;

            else res = stmp;

        }

        return res;

    }

    void init_map(map<char,string> &nmap)

    {

        nmap['1']="";nmap['2']="abc";nmap['3']="def";

        nmap['4']="ghi";nmap['5']="jkl";nmap['6']="mno";

        nmap['7']="pqrs";nmap['8']="tuv";nmap['9']="wxyz";

        nmap['0']=" ";

    }

};