【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;

    }

};