Smallest number containing all possible N length permutations using digits 0 to D

Given two integer N and D, the task is to find the size of the smallest string that contains all permutations of length N that can be formed using first D digits (0, 1, …, D-1).
Examples:

Input: N = 2, D = 2 
Output: 01100 
Explanation:
Possible permutations of length 2 from digits (0, 1) are {00, 01, 10, 11}. 
“01100” is one such string that contains all the permutations as a substring. 
Other possible answers are “00110”, “10011”, “11001”
Input: N = 2, D = 4 
Output: 03322312113020100 
Explaination:
Here all possible permutations of length 2 from digits {0, 1, 2, 3} are 
00 10 20 30 
01 11 21 31 
02 12 22 32 
03 13 23 33 
“03322312113020100” is a string of minimum length that contains all the above permutations.

Approach:
Append ‘0’ N-1 times and call DFS on the string in the current state. Append all the D characters one by one. Every time after appending, check if the new string is visited or not. If so, mark it visited by inserting it into a HashSet and add this character in the answer. Recursively call the DFS function on the last D characters. Repeat this process until all possible substrings of N length from the D digits are appended to the string. Print the final string generated.
Below is the implementation of the above approach: 

C++

// C++ Program to find the
// minimum length string
// consisting of all
// permutations of length N
// of D digits
#include <bits/stdc++.h>
using namespace std;

// Initialize set to see
// if all the possible
// permutations are present
// in the min length string
set<string> visited;

// To keep min length string
string ans;

// Generate the required string
void dfs(string curr, int D)
{
    // Iterate over all the possible
    // character
    for (int x = 0; x < D; ++x) {
        char chr = x + '0';

        // Append to make a new string
        string neighbour = curr + chr;
        // If the new string is not
        // visited

        if (visited.find(neighbour) == visited.end()) {

            // Add in set
            visited.insert(neighbour);
            // Call the dfs function on
            // the last d characters
            dfs(neighbour.substr(1), D);

            ans.push_back(chr);
        }
    }
}
string reqString(int N, int D)
{
    // Base case
    if (N == 1 && D == 1)
        return "0";

    visited.clear();
    ans.clear();
    string start;

    // Append '0' n-1 times
    for (int i = 0; i < N - 1; i++)
        start.append("0");

    // Call the DFS Function
    dfs(start, D);

    ans.append(start);
    return ans;
}

// Driver Code
int main()
{
    int N = 2;
    int D = 2;
    cout << reqString(N, D) << '\n';
    return 0;
}

Java

// Java Program to find the
// minimum length string
// consisting of all
// permutations of length N
// of D digits
import java.io.*;
import java.util.*;
import java.lang.*;

class GeeksforGeeks {

    // Initialize hashset to see
    // if all the possible
    // permutations are present
    // in the min length string
    static Set<String> visited;
    // To keep min length string
    static StringBuilder ans;

    public static String reqString(int N,
                                   int D)
    {
        // Base case
        if (N == 1 && D == 1)
            return "0";
        visited = new HashSet<>();
        ans = new StringBuilder();

        StringBuilder sb = new StringBuilder();
        // Append '0' n-1 times
        for (int i = 0; i < N - 1; ++i) {
            sb.append("0");
        }
        String start = sb.toString();
        // Call the DFS Function
        dfs(start, D);
        ans.append(start);

        return new String(ans);
    }
    // Generate the required string
    public static void dfs(String curr, int D)
    {
        // Iterate over all the possible
        // character
        for (int x = 0; x < D; ++x) {
            // Append to make a new string
            String neighbour = curr + x;
            // If the new string is not
            // visited
            if (!visited.contains(neighbour)) {
                // Add in hashset
                visited.add(neighbour);
                // Call the dfs function on
                // the last d characters
                dfs(neighbour.substring(1), D);

                ans.append(x);
            }
        }
    }
    // Driver Code
    public static void main(String args[])
    {

        int N = 2;
        int D = 2;
        System.out.println(reqString(N, D));
    }
}

Python3

# Python3 Program to find the
# minimum length string
# consisting of all
# permutations of length N
# of D digits

#  Initialize set to see
#  if all the possible
#  permutations are present
#  in the min length string
visited=set()

# To keep min length string
ans=[]

# Generate the required string
def dfs(curr, D):

    # Iterate over all possible character
    for c in range(D):
        c = str(c)

        # Append to make a new string
        neighbour = curr + c

        # If the new string is not visited
        if neighbour not in visited:

            # Add in set
            visited.add(neighbour)

            # Call the dfs function on the last d characters
            dfs(neighbour[1:], D)

            ans.append(c)

def reqString(N, D):
    # Base case
    if (N == 1 and D == 1):
        return "0"

    # Append '0' n-1 times
    start=''.join(['0']*(N - 1))

    # Call the DFS Function
    dfs(start, D)

    ans.extend(['0']*(N - 1))
    return ''.join(ans)

if __name__ == '__main__':
    N, D = 2, 2
    print(reqString(N,D))

    # This code is contributed by amartyaghoshgfg.

Javascript

<script>

// JavaScript Program to find the
// minimum length string
// consisting of all
// permutations of length N
// of D digits

// Initialize set to see
// if all the possible
// permutations are present
// in the min length string
let visited = new Set()

// To keep min length string
let ans=[]

// Generate the required string
function dfs(curr, D){

    // Iterate over all possible character
    for(let c = 0;c<D;c++){
        c = parseInt(c)

        // Append to make a new string
        let neighbour = curr + c

        // If the new string is not visited
        if(visited.has(neighbour) == false){

            // Add in set
            visited.add(neighbour)

            // Call the dfs function on the last d characters
            dfs(neighbour.substring(1), D)

            ans.push(c)
        }
    }

}

function reqString(N, D){
    // Base case
    if (N == 1 && D == 1)
        return "0"

    // Append '0' n-1 times
    let start=new Array(N - 1).fill('0').join('')

    // Call the DFS Function
    dfs(start, D)

    ans.push(new Array(N - 1).fill('0'))
    return ans.join('')
}

// driver code

let N = 2,D = 2
document.write(reqString(N,D))

// This code is contributed by shinjanpatra

</script>

Output:

01100

Time Complexity: O(N * D^N) 
Auxiliary Space: O(N * D^N)