Find Permutation of N numbers in range [1, N] such that K numbers have value same as their index

Given a positive integer N and an integer K such that 0 ≤ K ≤ N, the task is to find any permutation A of [1, N] such that the number of indices for which A_i = i is exactly K (1-based indexing). If there exists no such permutation, print −1.

Examples:

Input: N = 3, K = 1
Output: 1 3 2
Explanation: Consider the permutation A = [1, 3, 2]. We have A1=1, A2=3 and A3=2. 
So, this permutation has exactly 1 index such that A_i = i.

Input: N = 2, K = 1
Output: -1
Explanation : There are total 2 permutations of [1, 2] which are [1, 2] and [2, 1]. 
There are 2 indices in [1, 2] and 0 indices in [2, 1] for which A_i = i holds true. 
Thus, there doesn't exist any permutation of [1, 2] with exactly 1 index i for which Ai=i holds true.

Approach: The task can be solved using the greedy approach. Initially fix exactly K elements at their indices and then just randomly put N-K elements at different places. Follow the below steps to solve the problem:

1. Somehow fix K positions
2. Dislocate remaining N-K numbers
3. Cyclic shift remaining element by one

Below is the implementation of the above approach: 

// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;

// Function to print permutation
void permutation(int N, int K)
{
    if (K == N) {
        for (int i = 1; i <= N; i++) {
            cout << i << ' ';
        }
        cout << '\n';
    }
    else if (K == N - 1) {
        cout << "-1" << '\n';
    }
    else {
        for (int i = 1; i <= K; i++) {
            cout << i << ' ';
        }
        for (int i = K + 2; i <= N; i++) {
            cout << i << ' ';
        }
        cout << K + 1 << '\n';
    }
}

// Driver Code
int main()
{
    int N = 3;
    int K = 1;
    permutation(N, K);
    return 0;
}

// Java program to implement
// the above approach
import java.util.*;
public class GFG
{

// Function to print permutation
static void permutation(int N, int K)
{
    if (K == N) {
        for (int i = 1; i <= N; i++) {
            System.out.print(i + " ");
        }
        System.out.println();
    }
    else if (K == N - 1) {
        System.out.println("-1");
    }
    else {
        for (int i = 1; i <= K; i++) {
            System.out.print(i + " ");
        }
        for (int i = K + 2; i <= N; i++) {
            System.out.print(i + " ");
        }
        System.out.println(K + 1);
    }
}

// Driver Code
public static void main(String args[])
{
    int N = 3;
    int K = 1;
    permutation(N, K);
}
}

// This code is contributed by Samim Hossain Mondal.

# Python code for the above approach

# Function to print permutation
def permutation(N, K):
    if (K == N):
        for i in range(1, N + 1):
            print(i, end=" ")
        print('')
    elif (K == N - 1):
        print(-1)
    else:
        for i in range(1, K + 1):
            print(i, end=" ")
        for i in range(K + 2, N + 1):
            print(i, end=" ")
        print(K + 1)

# Driver Code
N = 3;
K = 1;
permutation(N, K);

# This code is contributed by Saurabh Jaiswal

// C# program to implement
// the above approach
using System;
public class GFG {

  // Function to print permutation
  static void permutation(int N, int K)
  {
    if (K == N) {
      for (int i = 1; i <= N; i++) {
        Console.Write(i + " ");
      }
      Console.WriteLine();
    }
    else if (K == N - 1) {
      Console.WriteLine("-1");
    }
    else {
      for (int i = 1; i <= K; i++) {
        Console.Write(i + " ");
      }
      for (int i = K + 2; i <= N; i++) {
        Console.Write(i + " ");
      }
      Console.Write(K + 1);
    }
  }

  // Driver Code
  public static void Main()
  {
    int N = 3;
    int K = 1;
    permutation(N, K);
  }
}

// This code is contributed by ukasp.

  <script>
        // JavaScript code for the above approach

        // Function to print permutation
        function permutation(N, K) {
            if (K == N) {
                for (let i = 1; i <= N; i++) {
                    document.write(i + " ")
                }
                document.write('<br>')
            }
            else if (K == N - 1) {
                document.write(-1 + '<br>')
            }
            else {
                for (let i = 1; i <= K; i++) {
                    document.write(i + " ")
                }
                for (let i = K + 2; i <= N; i++) {
                    document.write(i + " ")
                }
                document.write(K + 1 + '<br>')
            }
        }

        // Driver Code
        let N = 3;
        let K = 1;
        permutation(N, K);

  // This code is contributed by Potta Lokesh
    </script>

1 3 2

Time Complexity: O(N)
Auxiliary Space: O(1)

sauarbhyadav

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Article Tags :

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• Algo Geek
• DSA
• Arrays
• Algo-Geek 2021
• permutation

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