【leetcode】 Letter Combinations of a Phone Number(middle)

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.

思路：回溯

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        vector<string> lux(10);
        lux[2] = "abc"; lux[3] = "def"; lux[4] = "ghi"; lux[5] = "jkl";
        lux[6] = "mno"; lux[7] = "pqrs"; lux[8] = "tuv"; lux[9] = "wxyz";

        vector<string> ans;
        if(digits.empty())
        {
            ans.push_back("");
            return ans;
        }

        string X = digits;
        vector<int> S(digits.length());
        int k = 0;

        while(k >= 0)
        {
            while(k >= 0 && S[k] < lux[digits[k] - '0'].length())
            {
                X[k] = lux[digits[k] - '0'][S[k]++];
                if(k < digits.length() - 1)
                {
                    k++;
                }
                else
                {
                    ans.push_back(X); //当判断条件是 （长度 - 1）  时不需要 k-- 因为最后一位数字需要循环
                }
            }
            S[k] = 0;
            k--;
        }

        if(ans.empty())
        {
            ans.push_back("");
            return ans;
        }
        return ans;
    }
};