Letter Combinations of a Phone Number

class Solution {
private:
    vector<string> result;
    unordered_map<char, string> digitToLetters = {
        {'2', "abc"}, {'3', "def"},  {'4', "ghi"}, {'5', "jkl"},
        {'6', "mno"}, {'7', "pqrs"}, {'8', "tuv"}, {'9', "wxyz"}};
    string digitsInput;
    
public:
    vector<string> letterCombinations(string digits) {
        if (digits.empty()) {
            return result;
        }
        digitsInput = digits;
        combineLetters(0, "");
        return result;
    }
    
private:
    void combineLetters(int pos, string currentCombination) {
        if (currentCombination.length() == digitsInput.length()) {
            result.push_back(currentCombination);
            return;
        }
        string currentLetters = digitToLetters[digitsInput[pos]];
        for (char letter : currentLetters) {
            currentCombination.push_back(letter);
            combineLetters(pos + 1, currentCombination);
            currentCombination.pop_back();
        }
    }
};

The given C++ code defines a solution for generating all possible letter combinations that a given number string could represent on a phone keypad. Here's a detailed breakdown of how the code operates:

• An instance of unordered_map named digitToLetters maps each digit from '2' to '9' to its corresponding string of letters. This map is used to retrieve possible letter choices for phone digits.

• The class has a private member variable result which stores the final combinations of letters. Another private variable digitsInput holds the input digit string.

• The public member function letterCombinations is designed to initiate the process:

  • If the input string digits is empty, the function immediately returns the result, which will be empty.
  • The function sets digitsInput to the given digits then calls the recursive function combineLetters starting from position 0 with an empty string for accumulating combinations.

• The private function combineLetters performs the recursion:

  • It first checks if the length of the currentCombination string is equal to the length of digitsInput. If true, the current combination is complete and is added to result.
  • The function retrieves possible letters for the current digit position and iterates through each letter. The letter is added to the current combination, a recursive call is made to the next position, and the letter is then removed (backtracking) to explore other possibilities.

This method effectively combines backtracking with the use of a map to create an efficient solution for generating letter combinations based on-phone number mappings. This solution can handle any sequence of digits from '2' to '9', making it versatile for various input scenarios.