Print all possible words from phone digits
Last Updated : 28 Jan, 2025
Given a keypad represented by an integer array arr[] containing digits from [0, 9], the task is to print all possible letter combinations that the numbers could represent. A mapping of digits to letters (just like on the telephone buttons) is being followed.
Note: 0 and 1 do not map to any letters.
Examples:
Input: arr[] = [2, 3]
Output: [ad, ae, af, bd, be, bf, cd, ce, cf]
Input: arr[] = [2]
Output: [a, b, c]
[Approach 1] Using Recursion and Backtracking - O(4^n) Time and O(n) Space
The idea is to use recursion to generate all possible letter combinations for the given digits. Each digit maps to a set of letters, and the recursion builds combinations by appending letters for the current digit to a growing string (prefix). The base case is reached when the prefix length matches the size of the input array, at which point the combination is added to the result. If a digit is invalid (not in the range 2-9), it is skipped.
C++ // C++ implementation to print all possible // letter combinations using recursion #include <iostream> #include <vector> using namespace std; // Recursive function to generate combinations void possibleWordsRec(vector<int> &arr, int index, string &prefix, vector<string> &padMap, vector<string> &res) { // Base case: if the prefix length matches arr size if (index == arr.size()) { res.push_back(prefix); return; } // Get the corresponding digit int digit = arr[index]; // Skip invalid digits if (digit < 2 || digit > 9) { possibleWordsRec(arr, index + 1, prefix, padMap, res); return; } // Place all possible letters for this digit for (char ch : padMap[digit]) { prefix.push_back(ch); possibleWordsRec(arr, index + 1, prefix, padMap, res); prefix.pop_back(); } } // Function to find all possible letter combinations vector<string> possibleWords(vector<int> &arr) { vector<string> res; vector<string> padMap = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"}; string prefix = ""; possibleWordsRec(arr, 0, prefix, padMap, res); return res; } int main() { vector<int> arr = {2, 3}; vector<string> words = possibleWords(arr); for (string word : words) cout << word << " "; return 0; } // Java implementation to print all possible // letter combinations using recursion import java.util.ArrayList; class GfG { // Recursive function to generate combinations static void possibleWordsRec(int[] arr, int index, StringBuilder prefix, String[] padMap, ArrayList<String> res) { // Base case: if the prefix length matches arr size if (index == arr.length) { res.add(prefix.toString()); return; } // Get the corresponding digit int digit = arr[index]; // Skip invalid digits if (digit < 2 || digit > 9) { possibleWordsRec(arr, index + 1, prefix, padMap, res); return; } // Place all possible letters for this digit for (char ch : padMap[digit].toCharArray()) { prefix.append(ch); possibleWordsRec(arr, index + 1, prefix, padMap, res); prefix.deleteCharAt(prefix.length() - 1); } } // Function to find all possible letter combinations static ArrayList<String> possibleWords(int[] arr) { ArrayList<String> res = new ArrayList<>(); String[] padMap = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"}; StringBuilder prefix = new StringBuilder(); possibleWordsRec(arr, 0, prefix, padMap, res); return res; } public static void main(String[] args) { int[] arr = {2, 3}; ArrayList<String> words = possibleWords(arr); for (String word : words) System.out.print(word + " "); } } # Python implementation to print all possible # letter combinations using recursion # Recursive function to generate combinations def possibleWordsRec(arr, index, prefix, padMap, res): # Base case: if the prefix length matches arr size if index == len(arr): res.append(prefix) return # Get the corresponding digit digit = arr[index] # Skip invalid digits if digit < 2 or digit > 9: possibleWordsRec(arr, index + 1, prefix, padMap, res) return # Place all possible letters for this digit for ch in padMap[digit]: possibleWordsRec(arr, index + 1, prefix + ch, padMap, res) # Function to find all possible letter combinations def possibleWords(arr): res = [] padMap = ["", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"] possibleWordsRec(arr, 0, "", padMap, res) return res if __name__ == "__main__": arr = [2, 3] words = possibleWords(arr) for word in words: print(word, end=" ")
// C# implementation to print all possible // letter combinations using recursion using System; using System.Collections.Generic; class GfG { // Recursive function to generate combinations static void possibleWordsRec(int[] arr, int index, string prefix, string[] padMap, List<string> res) { // Base case: if the prefix length matches arr size if (index == arr.Length) { res.Add(prefix); return; } // Get the corresponding digit int digit = arr[index]; // Skip invalid digits if (digit < 2 || digit > 9) { possibleWordsRec(arr, index + 1, prefix, padMap, res); return; } // Place all possible letters for this digit foreach (char ch in padMap[digit]) { possibleWordsRec(arr, index + 1, prefix + ch, padMap, res); } } // Function to find all possible letter combinations static List<string> possibleWords(int[] arr) { List<string> res = new List<string>(); string[] padMap = { "", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz" }; possibleWordsRec(arr, 0, "", padMap, res); return res; } static void Main(string[] args) { int[] arr = { 2, 3 }; List<string> words = possibleWords(arr); foreach (string word in words) Console.Write(word + " "); } } // JavaScript implementation to print all possible // letter combinations using recursion // Recursive function to generate combinations function possibleWordsRec(arr, index, prefix, padMap, res) { // Base case: if the prefix length matches arr size if (index === arr.length) { res.push(prefix); return; } // Get the corresponding digit const digit = arr[index]; // Skip invalid digits if (digit < 2 || digit > 9) { possibleWordsRec(arr, index + 1, prefix, padMap, res); return; } // Place all possible letters for this digit for (let ch of padMap[digit]) { possibleWordsRec(arr, index + 1, prefix + ch, padMap, res); } } // Function to find all possible letter combinations function possibleWords(arr) { const res = []; const padMap = ["", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"]; possibleWordsRec(arr, 0, "", padMap, res); return res; } // Driver Code const arr = [2, 3]; const words = possibleWords(arr); console.log(words.join(" ")); Outputad ae af bd be bf cd ce cf
[Approach 2] Using Queue - O(4^n) Time and O(4^n) Space
The idea is to use a queue to iteratively generate all possible letter combinations for the given digits. Starting with an empty string in the queue, we repeatedly extend the current string by appending all possible letters corresponding to the next digit. If the string length matches the size of the input array, it is added to the result. This approach ensures all combinations are generated systematically. For implementation of this approach go to this post : Iterative Letter Combinations of a Phone Number
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