Leetcode123: 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 {public:    vector<string> letterCombinations(string digits) {        string str[] = {"abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};        int n = digits.size();        vector<string> res;        if(!n)            return res;        res.resize(1);        for(int i = 0; i < n; i++)        {            for(int j = res.size()-1; j >= 0; j--)            {                string s = str[digits[i]-'2'];                for(int k = s.size()-1; k>=0; k--)                {                    if(k)                        res.push_back(res[j]+s[k]);                    else                        res[j] += s[k];                }            }        }        return res;    }};