Problem description:

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”].

Solution:

This question is a relatively easy backtracking problem. The idea of backtracking is to find the end result of a branch, then move back one step and search for other results. 

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class Solution {
public:
    vector<string> dict{" ", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
    vector<string> letterCombinations(string digits) {
        vector<string> res;
        string tmp= "";
        helper(res, tmp, digits, 0);
        return res;
    }
    
    void helper(vector<string>& res, string tmp, string& digits, int index){
        if(index > digits.size()-1){
            res.push_back(tmp);
            return;
        }
        int num= digits[index]-'0';
        for(auto c: dict[num]){
            helper(res, tmp+c, digits, index+1);
        }
    }
};

reference:
https://goo.gl/o1acNw
https://goo.gl/HBP2MX