# 17. Letter Combinations of a Phone Number

Given a string containing digits from 2-9 inclusive, 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. Note that 1 does not map to any letters.

Example:

Input: "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

Approach 1: Backtracking.

# Code (Python)

Approach 1:

    def letterCombinations(self, digits: str) -> List[str]:
        table = {'2':'abc', '3':'def', '4':'ghi', '5':'jkl', '6':'mno', '7':'pqrs', '8':'tuv', '9':'wxyz'}
        if '0' in digits or not digits:
            return []
        result = ['']
        for number in digits:
            new_result = []
            for item in result:
                for letter in table[number]:
                    new_result.append(item + letter)
            result = new_result
        return result

# Code (C++)

Approach 1:

class Solution {
private:
    vector<string> letters = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
    void letterCombinations(string digits, string letterCom, vector<string>& letterComSet) {
        if (digits.size() == 0)
        {
            if (letterCom.size() > 0)
                letterComSet.push_back(letterCom);
            return;
        }
        int digit = digits[0] - '0';
        for (int i = 0; i < letters[digit].size(); ++i)
        {
            letterCombinations(
                digits.substr(1, digits.size() - 1),
                letterCom + letters[digit][i],
                letterComSet);
        }
    }
public:
    vector<string> letterCombinations(string digits) {
        vector<string> letterComSet;
        letterCombinations(digits, "", letterComSet);
        return letterComSet;
    }
};