LeetCode 17: Letter Combinations of a Phone Number

Difficulty: 3

Frequency 3

Problem:

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.

Solution:

class Solution {public:    string letters[10] = {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};    vector<string> letterCombinations(string digits) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        vector<string> answer;        vector<char> v_letters;        DFS (digits, 0, v_letters, answer);        return answer;    }    void DFS(string & digits, int i_level, vector<char> & v_letters, vector<string> & answer)    {        if (i_level==digits.size())        {            string str_letter;            for (int i = 0; i<v_letters.size(); i++)            {                str_letter += v_letters[i];            }            answer.push_back(str_letter);            return;        }                        for (int i = 0; i<letters[digits[i_level]-'0'].size(); i++)        {            v_letters.push_back(letters[digits[i_level]-'0'][i]);            DFS(digits, i_level+1, v_letters, answer);            v_letters.pop_back();        }    }};

Notes:

Just DFS, It is boring.