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.
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 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
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 class Solution { private: vector<string>mp = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"}; public: vector<string> letterCombinations(string digits) { vector<string> res; if(digits.empty()) return res; res.push_back(""); for(int i = 0; digits[i]; i++){ vector<string> tmp; for(int j = 0; mp[digits[i]'0'][j]; j++) for(int k = 0; k < res.size(); k++) tmp.push_back(res[k] + mp[digits[i]  '0'][j]); res = tmp; } return res; } };
