45-Letter Combinations of a Phone Number

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('');         //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']=" ";

    }
};