All combinations of strings that can be used to dial a number

2.8

Given a number, print all possible combinations of strings that can be used to dial the given number in a phone with following specifications.

In the given phone, we can dial,
2 using A or B or C,
3 using D or E or F,
……………….
8 using T or U or V,
9 using W or X or Y or Z,
1 using only 1
0 using 0.

For example if 23, is the given phone number, the program should print AD, AE, AF, BD, BE, BF, CD, CE, CF

The idea is to store digit to characters mapping in hash map. The map stores all characters that can be used dial a digit. We place every possible character for current digit and recur for remaining digits. Below is Java implementation of this idea.

// Java program to print all possible key strings
// that can be used to dial a phone number.
import java.util.HashMap;

class ConvertToString
{
    // A Recursive function to print all combinations
    // that can be used to dial a given number.
    // phNo ==> Given Phone Number
    // i    ==> Current digit of phNo to be processed
    // hm   ==> Stores characters that can be used to
    //          to dial a digit.
    // str  ==> Current output string
    static void printStrings(String phNo, int i,
                    HashMap<Character, String> hm,
                    StringBuilder str)
    {
        // If all digits are processed, print output
        // string
        if (i == phNo.length())
        {
            System.out.print(str + " ");
            return;
        }

        // Get current digit of phNo, and recur for all
        // characters that can be used to dial it.
        String s = hm.get(phNo.charAt(i));
        for (int j = 0; j < s.length(); j++)
        {
            str.append(s.charAt(j));
            printStrings(phNo, i+1, hm, str);
            str.deleteCharAt(str.length()-1);
        }
    }

    // Prints all possible combinations of strings that
    // can be used to dial c[].
    static void printStringForNumber(String phNo)
    {
        // Create a HashMap
        HashMap<Character, String> hm =
                    new HashMap<Character, String>();

        // For every digit, store characters that can
        // be used to dial it.
        hm.put('2', "ABC");
        hm.put('3', "DEF");
        hm.put('4', "GHI");
        hm.put('5', "JKL");
        hm.put('6', "MNO");
        hm.put('7', "PQRS");
        hm.put('8', "TUV");
        hm.put('9', "WXYZ");
        hm.put('1', "1");
        hm.put('0', "0");

        // Create a string to store a particular output
        // string
        StringBuilder str = new StringBuilder();

        // Call recursive function
        printStrings(phNo, 0, hm, str);
    }

    // Driver code to test above methods
    public static void main(String args[])
    {
        // Prints
        printStringForNumber("23");
    }
}

Output :

AD AE AF BD BE BF CD CE CF 

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