Print all possible strings of length k that can be formed from a set of n characters

Given a set of characters and a positive integer k, print all possible strings of length k that can be formed from the given set.

Examples:

Input: 
set[] = {'a', 'b'}, k = 3

Output:
aaa
aab
aba
abb
baa
bab
bba
bbb


Input: 
set[] = {'a', 'b', 'c', 'd'}, k = 1
Output:
a
b
c
d

For a given set of size n, there will be n^k possible strings of length k. The idea is to start from an empty output string (we call it prefix in following code). One by one add all characters to prefix. For every character added, print all possible strings with current prefix by recursively calling for k equals to k-1. 

Below is the implementation of above idea : 

C++

// C++ program to print all
// possible strings of length k
#include <bits/stdc++.h>
using namespace std;


// The main recursive method
// to print all possible
// strings of length k
void printAllKLengthRec(char set[], string prefix,
                                    int n, int k)
{

    // Base case: k is 0,
    // print prefix
    if (k == 0)
    {
        cout << (prefix) << endl;
        return;
    }

    // One by one add all characters
    // from set and recursively
    // call for k equals to k-1
    for (int i = 0; i < n; i++)
    {
        string newPrefix;

        // Next character of input added
        newPrefix = prefix + set[i];

        // k is decreased, because
        // we have added a new character
        printAllKLengthRec(set, newPrefix, n, k - 1);
    }

}

void printAllKLength(char set[], int k,int n)
{
    printAllKLengthRec(set, "", n, k);
}

// Driver Code
int main()
{

    cout << "First Test" << endl;
    char set1[] = {'a', 'b'};
    int k = 3;
    printAllKLength(set1, k, 2);

    cout << "Second Test\n";
    char set2[] = {'a', 'b', 'c', 'd'};
    k = 1;
    printAllKLength(set2, k, 4);
}

// This code is contributed
// by Mohit kumar

Java

// Java program to print all
// possible strings of length k

class GFG {

// The method that prints all
// possible strings of length k.
// It is mainly a wrapper over
// recursive function printAllKLengthRec()
static void printAllKLength(char[] set, int k)
{
    int n = set.length;
    printAllKLengthRec(set, "", n, k);
}

// The main recursive method
// to print all possible
// strings of length k
static void printAllKLengthRec(char[] set,
                               String prefix,
                               int n, int k)
{

    // Base case: k is 0,
    // print prefix
    if (k == 0)
    {
        System.out.println(prefix);
        return;
    }

    // One by one add all characters
    // from set and recursively
    // call for k equals to k-1
    for (int i = 0; i < n; ++i)
    {

        // Next character of input added
        String newPrefix = prefix + set[i];

        // k is decreased, because
        // we have added a new character
        printAllKLengthRec(set, newPrefix,
                                n, k - 1);
    }
}

// Driver Code
public static void main(String[] args)
{
    System.out.println("First Test");
    char[] set1 = {'a', 'b'};
    int k = 3;
    printAllKLength(set1, k);

    System.out.println("\nSecond Test");
    char[] set2 = {'a', 'b', 'c', 'd'};
    k = 1;
    printAllKLength(set2, k);
}
}

Python3

# Python 3 program to print all
# possible strings of length k

# The method that prints all
# possible strings of length k.
# It is mainly a wrapper over
# recursive function printAllKLengthRec()
def printAllKLength(set, k):

    n = len(set)
    printAllKLengthRec(set, "", n, k)

# The main recursive method
# to print all possible
# strings of length k
def printAllKLengthRec(set, prefix, n, k):

    # Base case: k is 0,
    # print prefix
    if (k == 0) :
        print(prefix)
        return

    # One by one add all characters
    # from set and recursively
    # call for k equals to k-1
    for i in range(n):

        # Next character of input added
        newPrefix = prefix + set[i]

        # k is decreased, because
        # we have added a new character
        printAllKLengthRec(set, newPrefix, n, k - 1)

# Driver Code
if __name__ == "__main__":

    print("First Test")
    set1 = ['a', 'b']
    k = 3
    printAllKLength(set1, k)

    print("\nSecond Test")
    set2 = ['a', 'b', 'c', 'd']
    k = 1
    printAllKLength(set2, k)

# This code is contributed
# by ChitraNayal

C#

// C# program to print all
// possible strings of length k
using System;

class GFG {

// The method that prints all
// possible strings of length k.
// It is mainly a wrapper over
// recursive function printAllKLengthRec()
static void printAllKLength(char[] set, int k)
{
    int n = set.Length;
    printAllKLengthRec(set, "", n, k);
}

// The main recursive method
// to print all possible
// strings of length k
static void printAllKLengthRec(char[] set,
                               String prefix,
                               int n, int k)
{

    // Base case: k is 0,
    // print prefix
    if (k == 0)
    {
        Console.WriteLine(prefix);
        return;
    }

    // One by one add all characters
    // from set and recursively
    // call for k equals to k-1
    for (int i = 0; i < n; ++i)
    {

        // Next character of input added
        String newPrefix = prefix + set[i];

        // k is decreased, because
        // we have added a new character
        printAllKLengthRec(set, newPrefix,
                                n, k - 1);
    }
}

// Driver Code
static public void Main ()
{
    Console.WriteLine("First Test");
    char[] set1 = {'a', 'b'};
    int k = 3;
    printAllKLength(set1, k);

    Console.WriteLine("\nSecond Test");
    char[] set2 = {'a', 'b', 'c', 'd'};
    k = 1;
    printAllKLength(set2, k);
}
}

// This code is contributed by Ajit.

Javascript

<script>
// Javascript program to print all
// possible strings of length k

    // The method that prints all
    // possible strings of length k.
    // It is mainly a wrapper over
    // recursive function printAllKLengthRec()
    function printAllKLength(set,k)
    {
        let n = set.length;
        printAllKLengthRec(set, "", n, k);
    }

    // The main recursive method
    // to print all possible
    // strings of length k
    function printAllKLengthRec(set,prefix,n,k)
    {
        // Base case: k is 0,
        // print prefix
        if (k == 0)
        {
            document.write(prefix+"<br>");
            return;
        }

        // One by one add all characters
        // from set and recursively
        // call for k equals to k-1
        for (let i = 0; i < n; ++i)
        {

            // Next character of input added
            let newPrefix = prefix + set[i];

            // k is decreased, because
            // we have added a new character
            printAllKLengthRec(set, newPrefix,
                                    n, k - 1);
        }
    }

    // Driver Code
    document.write("First Test<br>");
    let set1=['a', 'b'];
    let k = 3;
    printAllKLength(set1, k);

    document.write("<br>Second Test<br>");
    let set2 = ['a', 'b', 'c', 'd'];
    k = 1;
    printAllKLength(set2, k);

    // This code is contributed by avanitrachhadiya2155

</script>

Output:

First Test
aaa
aab
aba
abb
baa
bab
bba
bbb

Second Test
a
b
c
d

Time complexity: O(n^k)

Auxiliary Space: O(k)

The above solution is mainly a generalization of this post.

This article is contributed by Abhinav Ramana. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above