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Find permutation of first N natural numbers that satisfies the given condition
  • Last Updated : 09 Apr, 2021

Given two integer N and K, the task is to find the permutation P of first N natural numbers such that there are exactly K elements which satisfies the condition GCD(P[i], i) > 1 for all 1 ≤ i ≤ N.
Examples: 
 

Input: N = 3, K = 1 
Output: 2 1 3 
GCD(P[1], 1) = GCD(2, 1) = 1 
GCD(P[2], 2) = GCD(1, 2) = 1 
GCD(P[3], 3) = GCD(3, 3) = 3 
There is exactly 1 element such that GCD(P[i], i) > 1
Input: N = 5, K = 2 
Output: 3 1 2 4 5 
 

 

Approach: Keep the last K elements in their place. The rest of the elements are moved such that ith element is placed in (i + 1)th position and (N – K)th element is kept in position 1 because gcd(x, x + 1) = 1.
Below is the implementation of the above approach: 
 

C++




// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;
 
// Function to find permutation(p) of first N
// natural numbers such that there are exactly K
// elements of permutation such that GCD(p[i], i)>1
void Permutation(int n, int k)
{
    int p[n + 1];
 
    // First place all the numbers
    // in their respective places
    for (int i = 1; i <= n; i++)
        p[i] = i;
 
    // Modify for first n-k integers
    for (int i = 1; i < n - k; i++)
        p[i + 1] = i;
 
    // In first index place n-k
    p[1] = n - k;
 
    // Print the permutation
    for (int i = 1; i <= n; i++)
        cout << p[i] << " ";
}
 
// Driver code
int main()
{
    int n = 5, k = 2;
    Permutation(n, k);
 
    return 0;
}

Java




// Java implementation of the approach
 
class GFG
{
     
    // Function to find permutation(p) of first N
    // natural numbers such that there are exactly K
    // elements of permutation such that GCD(p[i], i)>1
    static void Permutation(int n, int k)
    {
        int[] p = new int[n + 1];
 
        // First place all the numbers
        // in their respective places
        for (int i = 1; i <= n; i++)
            p[i] = i;
 
        // Modify for first n-k integers
        for (int i = 1; i < n - k; i++)
            p[i + 1] = i;
 
        // In first index place n-k
        p[1] = n - k;
 
        // Print the permutation
        for (int i = 1; i <= n; i++)
            System.out.print(p[i] + " ");
    }
 
    // Driver code
    public static void main(String[] args)
    {
        int n = 5, k = 2;
        Permutation(n, k);
    }
}
 
// This code is contributed by Naman_Garg

Python3




# Python 3 implementation of the approach
 
# Function to find permutation(p)
# of first N natural numbers such that
# there are exactly K elements of
# permutation such that GCD(p[i], i)>1
def Permutation(n, k):
    p = [0 for i in range(n + 1)]
 
    # First place all the numbers
    # in their respective places
    for i in range(1, n + 1):
        p[i] = i
 
    # Modify for first n-k integers
    for i in range(1, n - k):
        p[i + 1] = i
 
    # In first index place n-k
    p[1] = n - k
 
    # Print the permutation
    for i in range(1, n + 1):
        print(p[i], end = " ")
 
# Driver code
if __name__ == '__main__':
    n = 5
    k = 2
    Permutation(n, k)
     
# This code is contributed by
# Surendra_Gangwar

C#




// C# implementation of the approach
using System;
 
class GFG
{
    // Function to find permutation(p) of first N
    // natural numbers such that there are exactly K
    // elements of permutation such that GCD(p[i], i)>1
    static void Permutation(int n, int k)
    {
        int[] p = new int[n + 1];
 
        // First place all the numbers
        // in their respective places
        for (int i = 1; i <= n; i++)
            p[i] = i;
 
        // Modify for first n-k integers
        for (int i = 1; i < n - k; i++)
            p[i + 1] = i;
 
        // In first index place n-k
        p[1] = n - k;
 
        // Print the permutation
        for (int i = 1; i <= n; i++)
            Console.Write(p[i] + " ");
    }
 
    // Driver code
    static public void Main ()
    {
        int n = 5, k = 2;
        Permutation(n, k);
    }
}
 
// This code is contributed by ajit.

PHP




<?php
// PHP implementation of the approach
 
// Function to find permutation(p) of first N
// natural numbers such that there are exactly K
// elements of permutation such that GCD(p[i], i)>1
function Permutation($n, $k)
{
    $p = array();
 
    // First place all the numbers
    // in their respective places
    for ($i = 1; $i <= $n; $i++)
        $p[$i] = $i;
 
    // Modify for first n-k integers
    for ($i = 1; $i < $n - $k; $i++)
        $p[$i + 1] = $i;
 
    // In first index place n-k
    $p[1] = $n - $k;
 
    // Print the permutation
    for ($i = 1; $i <= $n; $i++)
        echo $p[$i], " ";
}
 
// Driver code
$n = 5; $k = 2;
Permutation($n, $k);
 
// This code is contributed by AnkitRai01
?>

Javascript




<script>
//Javascript implementation of the approach
 
// Function to find permutation(p) of first N
// natural numbers such that there are exactly K
// elements of permutation such that GCD(p[i], i)>1
function Permutation(n, k)
{
    let p  = new Array(n + 1);
 
    // First place all the numbers
    // in their respective places
    for (let i = 1; i <= n; i++)
        p[i] = i;
 
    // Modify for first n-k integers
    for (let i = 1; i < n - k; i++)
        p[i + 1] = i;
 
    // In first index place n-k
    p[1] = n - k;
 
    // Print the permutation
    for (let i = 1; i <= n; i++)
        document.write(p[i] + " ");
}
 
// Driver code
    let n = 5, k = 2;
    Permutation(n, k);
  
// This code is contributed by Mayank Tyagi
</script>
Output: 
3 1 2 4 5

 

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