Generate a random permutation of 1 to N

Given an integer N, the task is to generate N non repeating random numbers.

Examples:

Input: N = 5
Output: 1 5 2 4 3

Input: N = 8
Output: 7 2 1 8 3 6 4 5

Approach: Create an array of N elements and initialize the elements as 1, 2, 3, 4, …, N then shuffle the array elements using Fisher–Yates shuffle Algorithm.

Fisher–Yates shuffle Algorithm works in O(n) time complexity. The assumption here is, we are given a function rand() that generates random number in O(1) time.
The idea is to start from the last element, swap it with a randomly selected element from the whole array (including last). Now consider the array from 0 to n-2 (size reduced by 1), and repeat the process till we hit the first element.

Below is the implementation of the above approach:

C++

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// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;
  
// Function to return the next random number
int getNum(vector<int>& v)
{
  
    // Size of the vector
    int n = v.size();
  
    // Generate a random number
    srand(time(NULL));
  
    // Make sure the number is within
    // the index range
    int index = rand() % n;
  
    // Get random number from the vector
    int num = v[index];
  
    // Remove the number from the vector
    swap(v[index], v[n - 1]);
    v.pop_back();
  
    // Return the removed number
    return num;
}
  
// Function to generate n non-repeating random numbers
void generateRandom(int n)
{
    vector<int> v(n);
  
    // Fill the vector with the values
    // 1, 2, 3, ..., n
    for (int i = 0; i < n; i++)
        v[i] = i + 1;
  
    // While vector has elements
    // get a random number from the vector and print it
    while (v.size()) {
        cout << getNum(v) << " ";
    }
}
  
// Driver code
int main()
{
    int n = 8;
    generateRandom(n);
  
    return 0;
}

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Java

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// Java implementation of the approach 
import java.util.*;
import java.lang.Math;
  
class GfG
{
  
    // Function to return the next random number 
    static int getNum(ArrayList<Integer> v) 
    
        // Size of the vector 
        int n = v.size(); 
      
        // Make sure the number is within 
        // the index range 
        int index = (int)(Math.random() * n); 
      
        // Get random number from the vector 
        int num = v.get(index); 
      
        // Remove the number from the vector 
        v.set(index, v.get(n - 1));
        v.remove(n - 1); 
      
        // Return the removed number 
        return num; 
    
      
    // Function to generate n 
    // non-repeating random numbers 
    static void generateRandom(int n) 
    
        ArrayList<Integer> v = new ArrayList<>(n); 
      
        // Fill the vector with the values 
        // 1, 2, 3, ..., n 
        for (int i = 0; i < n; i++) 
            v.add(i + 1); 
      
        // While vector has elements 
        // get a random number from the vector and print it 
        while (v.size() > 0
        
            System.out.print(getNum(v) + " "); 
        
    
  
    // Driver code
    public static void main(String []args)
    {
          
        int n = 8
        generateRandom(n); 
    }
}
  
// This code is contributed by Rituraj Jain 

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Python3

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# Python3 implementation of the 
# above approach 
  
# import random module
import random
  
# Function to return the next 
# random number 
def getNum(v) :
  
    # Size of the vector 
    n = len(v)
  
    # Generate a random number within
    # the index range
    index = random.randint(0, n - 1)
  
    # Get random number from the vector 
    num = v[index] 
  
    # Remove the number from the vector 
    v[index], v[n - 1] = v[n - 1], v[index]
    v.pop() 
  
    # Return the removed number 
    return num
  
# Function to generate n non-repeating 
# random numbers 
def generateRandom(n) :
      
    v = [0] * n
  
    # Fill the vector with the values 
    # 1, 2, 3, ..., n 
    for i in range(n) : 
        v[i] = i + 1
  
    # While vector has elements get a  
    # random number from the vector 
    # and print it 
    while (len(v)) :
        print(getNum(v), end = " "
  
# Driver code 
if __name__ == "__main__" :
      
    n = 8
    generateRandom(n)
      
# This code is contributed by Ryuga

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PHP

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<?php 
// PHP implementation of the approach
  
// Function to return the next random number
function getNum(&$v)
{
  
    // Size of the vector
    $n = sizeof($v);
  
    // Generate a random number
    srand(time(NULL));
  
    // Make sure the number is within
    // the index range
    $index = rand() % $n;
  
    // Get random number from the vector
    $num = $v[$index];
  
    // Remove the number from the vector
    $t = $v[$index];
    $v[$index] = $v[$n - 1];
    $v[$n - 1] = $t;
    array_pop($v);
  
    // Return the removed number
    return $num;
}
  
// Function to generate n non-repeating
// random numbers
function generateRandom($n)
{
    $v = array(0, $n, NULL);
  
    // Fill the vector with the values
    // 1, 2, 3, ..., n
    for ($i = 0; $i < $n; $i++)
        $v[$i] = $i + 1;
  
    // While vector has elements
    // get a random number from the
    // vector and print it
    while (sizeof($v)) 
    {
        echo getNum($v) . " ";
    }
}
  
// Driver code
$n = 8;
generateRandom($n);
  
// This code is contributed by ita_c
?>

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Output:

3 4 5 8 6 2 1 7


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Improved By : AnkitRai01, Ita_c, rituraj_jain



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