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

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

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++

// 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; 
} 

Java

// 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  

Python3

# 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 

PHP

<?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 
?> 

Output:

3 4 5 8 6 2 1 7

Don’t stop now and take your learning to the next level. Learn all the important concepts of Data Structures and Algorithms with the help of the most trusted course: DSA Self Paced. Become industry ready at a student-friendly price.

My Personal Notes

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

C++

Java

Python3

PHP

Recommended Posts: