Periodic Binary String With Minimum Period and a Given Binary String as Subsequence.

Periodic Binary String: A Binary string is called periodic if it can be written as a repetition of a binary string of smaller or same length. For example, 101010 is a periodic binary string with period 10 as we can get the string by repeatedly appending 10 to itself. In general, the string S with period P means, S_i is equal to S_{i + P}. 

Problem: Given a binary string S, the task is to find the periodic string with minimum possible period with the following additional conditions 
1) The given string S should be a subsequence of the result 
2) Length of the result should not be more than twice the length of input string.

Examples:

Input:S = “01” 
Output:0101 
Explanation:The output string has period as 2 and has given string as subsequence.

Input :S = “111” 
Output :111 
Explanation:The output string has period as 1.

Input:S = “110” 
Output: 101010 
Explanation:The output string has period as 2 and has given string as subsequence. Please note that 110110 is not answer because the period length is more.

Approach:
The main idea depends on two possibilities:

1. If the string consists all 1s or all 0s, the answer is the given string S itself having period as 1.
2. If the string consists of dissimilar elements, find the string with period 2 and having length as twice the length of given string S

Below is the implementation of the above approach: 

C++

// C++ implementation to find the
// periodic string with minimum period
#include <bits/stdc++.h>
using namespace std;

// Function to find the periodic string
// with minimum period
void findPeriodicString(string S)
{
    int l = 2 * S.length();

    int count = 0;
    for (int i = 0; i < S.length(); i++) {
        if (S[i] == '1')
            count++;
    }

    // Print the string S if it
    // consists of similar elements
    if (count == S.length() || count == 0)
        cout << S << "\n";

    // Find the required periodic
    // string with period 2
    else {
        char arr[l];
        for (int i = 0; i < l; i += 2) {
            arr[i] = '1';
            arr[i + 1] = '0';
        }

        for (int i = 0; i < l; i++)
            cout << arr[i];
        cout << "\n";
    }
}

// Driver Code
int main()
{
    string S = "1111001";
    findPeriodicString(S);
    return 0;
}

Java

// Java implementation to find the
// periodic string with minimum period
class GFG{

// Function to find the periodic string
// with minimum period
static void findPeriodicString(String S)
{
    int l = 2 * S.length();
    int count = 0;

    for(int i = 0; i < S.length(); i++)
    {
        if (S.charAt(i) == '1')
            count++;
    }

    // Print the string S if it
    // consists of similar elements
    if (count == S.length() || count == 0)
        System.out.println(S);

    // Find the required periodic
    // string with period 2
    else
    {
        char arr[] = new char[l];
        for(int i = 0; i < l; i += 2)
        {
            arr[i] = '1';
            arr[i + 1] = '0';
        }

        for(int i = 0; i < l; i++)
            System.out.print(arr[i]);
        System.out.println();
    }
}

// Driver Code
public static void main (String[] args)
{
    String S = "1111001";

    findPeriodicString(S);
}
}

// This code is contributed by chitranayal

Python3

# Python3 implementation to find the
# periodic with minimum period

# Function to find the periodic string
# with minimum period
def findPeriodicString(S):
    l = 2 * len(S)

    count = 0
    for i in range(len(S)):
        if (S[i] == '1'):
            count += 1

    # Print the S if it
    # consists of similar elements
    if (count == len(S) or count == 0):
        print(S)

    # Find the required periodic
    # with period 2
    else:
        arr = ['0']*l
        for i in range(0, l, 2):
            arr[i] = '1'
            arr[i + 1] = '0'

        for i in range(l):
            print(arr[i],end="")

# Driver Code
if __name__ == '__main__':
    S = "1111001"
    findPeriodicString(S)

# This code is contributed by mohit kumar 29

C#

// C# implementation to find the
// periodic string with minimum period
using System;

class GFG{

// Function to find the periodic string
// with minimum period
static void findPeriodicString(string S)
{
    int l = 2 * S.Length;
    int count = 0;

    for(int i = 0; i < S.Length; i++)
    {
        if (S[i] == '1')
            count++;
    }

    // Print the string S if it
    // consists of similar elements
    if (count == S.Length || count == 0)
        Console.WriteLine(S);

    // Find the required periodic
    // string with period 2
    else
    {
        char[] arr = new char[l];
        for(int i = 0; i < l; i += 2)
        {
            arr[i] = '1';
            arr[i + 1] = '0';
        }

        for(int i = 0; i < l; i++)
            Console.Write(arr[i]);

        Console.WriteLine();
    }
}

// Driver code
public static void Main ()
{
    string S = "1111001";

    findPeriodicString(S);
}
}

// This code is contributed by sanjoy_62

Javascript

<script>

// Javascript implementation to find the
// periodic string with minimum period

// Function to find the periodic string
// with minimum period
function findPeriodicString(S)
{
    let l = 2 * S.length;
    let count = 0;

    for(let i = 0; i < S.length; i++)
    {
        if (S[i] == '1')
            count++;
    }

    // Print the string S if it
    // consists of similar elements
    if (count == S.length || count == 0)
        document.write(S + "<br>");

    // Find the required periodic
    // string with period 2
    else
    {
        let arr = new Array(l);
        for(let i = 0; i < l; i += 2)
        {
            arr[i] = '1';
            arr[i + 1] = '0';
        }

        for(let i = 0; i < l; i++)
            document.write(arr[i]);

        document.write("<br>");
    }
}

// Driver Code
let S = "1111001";
findPeriodicString(S);

// This code is contributed by avanitrachhadiya2155

</script>

Output:

10101010101010

Time Complexity: O(N)
Auxiliary Space: O(2*N)