# 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 ` `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 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 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

 ` `

Output:

```3 4 5 8 6 2 1 7
```

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