Number of shuffles required for each element to return to its initial position

Given an integer array arr[] containing a permutation of integers from 1 to N. Let K[] be some arbitrary array. Every element in the array arr[i] represents the index of the element to which the element which is initially at the position ‘i’ in the array K[] is placed. The task is to find the number of shuffles needed to be performed on K[] to get back the elements to the initial position.

Note: It is given that at least 1 shuffle has to be made over K[] (i.e.), the element which is initially at the position ‘i’ in the array K[] has to be placed in the index arr[i] at least once for all the elements in the array K[].

Examples:

Input: arr[] = {3, 4, 1, 2}
Output: 2 2 2 2
Explanation:
Let the initial array B[] = {5, 6, 7, 8}. Therefore:
After 1st shuffle: The first element will go to the third position, the second element will go to the fourth position. Therefore, B[] = {7, 8, 5, 6}.
After 2nd shuffle: The first element will go to the third position, the second element will go to the fourth position. Therefore, B[] = {5, 6, 7, 8}.
Therefore, the number of shuffles is 2 for all the elements to get back to the same initial position.

Input : arr[] = {4, 6, 2, 1, 5, 3}
Output : 2 3 3 2 1 3



Naive Approach: The naive approach for this problem is to count the number of shuffles needed for every element. The time complexity for this approach would be O(N2).

Efficient Approach: An efficient approach for this problem is to first calculate the number of cycles in the problem. The elements in the same cycle have the same number of shuffles. For example, in the given array arr[] = {3, 4, 1, 2}:

  • At every shuffle, the first element always goes to the third place and the third element always comes to the first place.
  • Therefore, it can be concluded that both the elements are in a cycle. Therefore, the number of shuffles taken for both the elements is 2 irrespective of what the array K[] is.
  • Therefore, the idea is to find the number of such cycles in the array and skip those elements.

Below is the implementation of the above approach:

C++

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// C++ program to find the number of
// shuffles required for each element
// to return to its initial position
  
#include <bits/stdc++.h>
using namespace std;
  
// Function to count the number of
// shuffles required for each element
// to return to its initial position
void countShuffles(int* A, int N)
{
    // Initialize array to store
    // the counts
    int count[N];
  
    // Initialize visited array to
    // check if visited before or not
    bool vis[N];
    memset(vis, false, sizeof(vis));
  
    // Making the array 0-indexed
    for (int i = 0; i < N; i++)
        A[i]--;
  
    for (int i = 0; i < N; i++) {
        if (!vis[i]) {
  
            // Initialize vector to store the
            // elements in the same cycle
            vector<int> cur;
  
            // Initialize variable to store
            // the current element
            int pos = i;
  
            // Count number of shuffles
            // for current element
            while (!vis[pos]) {
  
                // Store the elements in
                // the same cycle
                cur.push_back(pos);
  
                // Mark visited
                vis[pos] = true;
  
                // Make the shuffle
                pos = A[pos];
            }
  
            // Store the result for all the
            // elements in the same cycle
            for (auto el : cur)
                count[el] = cur.size();
        }
    }
  
    // Print the result
    for (int i = 0; i < N; i++)
        cout << count[i] << " ";
    cout << endl;
}
  
// Driver code
int main()
{
    int arr[] = { 4, 6, 2, 1, 5, 3 };
  
    int N = sizeof(arr) / sizeof(arr[0]);
  
    countShuffles(arr, N);
  
    return 0;
}

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Java

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// Java program to find the number of
// shuffles required for each element
// to return to its initial position
import java.util.*;
  
class GFG {
  
// Function to count the number of
// shuffles required for each element
// to return to its initial position
static void countShuffles(int[] A, int N)
{
      
    // Initialize array to store
    // the counts
    int[] count = new int[N];
  
    // Initialize visited array to
    // check if visited before or not
    boolean[] vis = new boolean[N];
  
    // Making the array 0-indexed
    for(int i = 0; i < N; i++)
       A[i]--;
  
    for(int i = 0; i < N; i++)
    {
       if (!vis[i])
       {
             
           // Initialize vector to store the
           // elements in the same cycle
           Vector<Integer> cur = new Vector<>();  
             
           // Initialize variable to store
           // the current element
           int pos = i;
             
           // Count number of shuffles
           // for current element
           while (!vis[pos])
           {
                 
               // Store the elements in
               // the same cycle
               cur.add(pos);
                 
               // Mark visited
               vis[pos] = true;
                 
               // Make the shuffle
               pos = A[pos];
            }
              
            // Store the result for all the
            // elements in the same cycle
            for(int k = 0; k < cur.size(); k++)
               count[cur.get(k)] = cur.size();
       }
    }
      
    // Print the result
    for(int k = 0; k < N; k++)
       System.out.print(count[k] + " ");
}
  
// Driver code
public static void main(String[] args)
{
    int arr[] = { 4, 6, 2, 1, 5, 3 };
    int N = arr.length;
  
    countShuffles(arr, N);
}
}
  
// This code is contributed by offbeat

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Python3

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# Python3 program to find the number of
# shuffles required for each element
# to return to its initial position
  
# Function to count the number of
# shuffles required for each element
# to return to its initial position
def countShuffles(A, N):
  
    # Initialize array to store
    # the counts
    count = [0] * N
  
    # Initialize visited array to
    # check if visited before or not
    vis = [False] * N
  
    # Making the array 0-indexed
    for i in range(N):
        A[i] -= 1
  
    for i in range(N):
        if (not vis[i]):
  
            # Initialize vector to store the
            # elements in the same cycle
            cur = []
  
            # Initialize variable to store
            # the current element
            pos = i
  
            # Count number of shuffles
            # for current element
            while (not vis[pos]):
  
                # Store the elements in
                # the same cycle
                cur.append(pos)
  
                # Mark visited
                vis[pos] = True
  
                # Make the shuffle
                pos = A[pos]
  
            # Store the result for all the
            # elements in the same cycle
            for el in cur:
                count[el] = len(cur)
  
    # Print the result
    for i in range(N):
        print(count[i], end = " ")
    print()
  
# Driver code
if __name__ == "__main__":
      
    arr = [ 4, 6, 2, 1, 5, 3 ]
    N = len(arr)
    countShuffles(arr, N)
      
# This code is contributed by chitranayal

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C#

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// C# program to find the number of
// shuffles required for each element
// to return to its initial position
using System;
using System.Collections;
  
class GFG{
  
// Function to count the number of
// shuffles required for each element
// to return to its initial position
static void countShuffles(int[] A, int N)
{
      
    // Initialize array to store
    // the counts
    int[] count = new int[N];
  
    // Initialize visited array to
    // check if visited before or not
    bool[] vis = new bool[N];
  
    // Making the array 0-indexed
    for(int i = 0; i < N; i++)
        A[i]--;
  
    for(int i = 0; i < N; i++)
    {
        if (!vis[i])
        {
              
            // Initialize vector to store the
            // elements in the same cycle
            ArrayList cur = new ArrayList(); 
          
            // Initialize variable to store
            // the current element
            int pos = i;
                  
            // Count number of shuffles
            // for current element
            while (!vis[pos])
            {
                  
                // Store the elements in
                // the same cycle
                cur.Add(pos);
                      
                // Mark visited
                vis[pos] = true;
                      
                // Make the shuffle
                pos = A[pos];
            }
              
            // Store the result for all the
            // elements in the same cycle
            for(int k = 0; k < cur.Count; k++)
                count[(int)cur[k]] = cur.Count;
        }
    }
      
    // Print the result
    for(int k = 0; k < N; k++)
        Console.Write(count[k] + " ");
}
  
// Driver code
public static void Main(string[] args)
{
    int []arr = { 4, 6, 2, 1, 5, 3 };
    int N = arr.Length;
  
    countShuffles(arr, N);
}
}
  
// This code is contributed by rutvik_56

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Output:

2 3 3 2 1 3

Time Complexity: O(N), where N is the size of the array.

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Improved By : chitranayal, offbeat, rutvik_56

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