Iterative Letter Combinations of a Phone Number

Given an integer array containing digits from [0, 9], the task is to print all possible letter combinations that the numbers could represent.
A mapping of digit to letters (just like on the telephone buttons) is being followed. Note that 0 and 1 do not map to any letters. All the mapping are shown in the image below: Example:

Input: arr[] = {2, 3}
Output: ad ae af bd be bf cd ce cf

Input: arr[] = {9}
Output: w x y z

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Approach: Now let us think how we would approach this problem without doing it in an iterative way. A recursive solution is intuitive and common. We keep adding each possible letter recursively and this will generate all the possible strings.

Let us think about how we can build an iterative solution using the recursive one. Recursion is possible through the use of a stack. So if we use a stack instead of a recursive function will that be an iterative solution? One could say so speaking technically but we then aren’t really doing anything different in terms of logic.

A Stack is a LIFO DS. Can we use another Data structure? What will be the difference if we use a FIFO DS? Let’s say a queue. Since BFS is done by queue and DFS by stack is there any difference between the two?

The difference between DFS and BFS is similar to this question. In DFS we will find each path possible in the tree one by one. It will perform all steps for a path first whereas BFS will build all paths together one step at a time.

So, a queue would work perfectly for this question. The only difference between the two algorithms using queue and stack will be the way in which they are formed. Stack will form all strings completely one by one whereas the queue will form all the strings together i.e. after x number of passes all the strings will have a length of x.

Below is the implementation of the above approach:

C++

 // C++ implementation of the approach #include using namespace std;    // Function to return a vector that contains // all the generated letter combinations vector letterCombinationsUtil(const int number[],                                       int n,                                       const string table[]) {     // To store the generated letter combinations     vector list;        queue q;     q.push("");        while (!q.empty()) {         string s = q.front();         q.pop();            // If complete word is generated         // push it in the list         if (s.length() == n)             list.push_back(s);         else                // Try all possible letters for current digit             // in number[]             for (auto letter : table[number[s.length()]])                 q.push(s + letter);     }        // Return the generated list     return list; }    // Function that creates the mapping and // calls letterCombinationsUtil void letterCombinations(const int number[], int n) {        // table[i] stores all characters that     // corresponds to ith digit in phone     string table         = { "", "", "abc", "def", "ghi", "jkl",             "mno", "pqrs", "tuv", "wxyz" };        vector list         = letterCombinationsUtil(number, n, table);        // Print the contents of the vector     for (auto word : list)         cout << word << " ";        return; }    // Driver program int main() {     int number[] = { 2, 3 };     int n = sizeof(number) / sizeof(number);        letterCombinations(number, n);        return 0; }

Java

 // Java implementation of the approach import java.io.*; import java.util.*;    class GFG  {     // Function to return a vector that contains      // all the generated letter combinations     static ArrayList letterCombinationsUtil(int[] number, int n,                                                              String[] table)     {             // To store the generated letter combinations             ArrayList list = new ArrayList<>();                            Queue q = new LinkedList<>();             q.add("");                        while(!q.isEmpty())              {                     String s = q.remove();                        // If complete word is generated                      // push it in the list                     if (s.length() == n)                     list.add(s);                     else                      {                         String val = table[number[s.length()]];                         for (int i = 0; i < val.length(); i++)                         {                             q.add(s + val.charAt(i));                         }                     }              }             return list;     }         // Function that creates the mapping and      // calls letterCombinationsUtil      static void letterCombinations(int[] number, int n)     {             // table[i] stores all characters that              // corresponds to ith digit in phone             String[] table = { "", "", "abc", "def", "ghi", "jkl",              "mno", "pqrs", "tuv", "wxyz" };                 ArrayList list =                          letterCombinationsUtil(number, n, table);                            // Print the contents of the list             for (int i = 0; i < list.size(); i++)             {                 System.out.print(list.get(i) + " ");             }     }        // Driver code     public static void main(String args[])     {             int[] number = { 2, 3 };             int n = number.length;             letterCombinations(number, n);      } }     // This code is contributed by rachana soma

Python

 # Python3 implementation of the approach from collections import deque    # Function to return a list that contains  # all the generated letter combinations def letterCombinationsUtil(number, n, table):        list = []     q = deque()     q.append("")        while len(q) != 0:         s = q.pop()            # If complete word is generated         # push it in the list         if len(s) == n:             list.append(s)         else:                # Try all possible letters for current digit             # in number[]             for letter in table[number[len(s)]]:                 q.append(s + letter)        # Return the generated list     return list       # Function that creates the mapping and # calls letterCombinationsUtil def letterCombinations(number, n):        # table[i] stores all characters that      # corresponds to ith digit in phone     table = ["", "", "abc", "def", "ghi", "jkl",             "mno", "pqrs", "tuv", "wxyz"]        list = letterCombinationsUtil(number, n, table)        s = ""     for word in list:         s += word + " "        print(s)     return       # Driver program number = [2, 3] n = len(number)    letterCombinations(number, n)

C#

 // C# implementation of the approach using System; using System.Collections.Generic;     class GFG  { // Function to return a vector that contains  // all the generated letter combinations static List letterCombinationsUtil(int[] number,                                             int n, String[] table) {     // To store the generated letter combinations     List list = new List();            Queue q = new Queue();     q.Enqueue("");        while(q.Count != 0)      {         String s = q.Dequeue();            // If complete word is generated          // push it in the list         if (s.Length == n)         list.Add(s);         else         {             String val = table[number[s.Length]];             for (int i = 0; i < val.Length; i++)             {                 q.Enqueue(s + val[i]);             }         }      }     return list; }     // Function that creates the mapping and  // calls letterCombinationsUtil  static void letterCombinations(int[] number, int n) {     // table[i] stores all characters that      // corresponds to ith digit in phone     String[] table = { "", "", "abc", "def", "ghi", "jkl",                             "mno", "pqrs", "tuv", "wxyz" };         List list =           letterCombinationsUtil(number, n, table);            // Print the contents of the list     for (int i = 0; i < list.Count; i++)     {         Console.Write(list[i] + " ");     } }    // Driver code public static void Main(String []args) {     int[] number = { 2, 3 };     int n = number.Length;     letterCombinations(number, n);  } }     // This code is contributed by Princi Singh

Output:

ad ae af bd be bf cd ce cf

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Improved By : rachana soma, princi singh