# Check if an Array is a permutation of numbers from 1 to N

Given an array arr containing N positive integers, the task is to check if the given array arr represents a permutation or not.

A sequence of N integers is called a permutation if it contains all integers from 1 to N exactly once.

Examples:

Input: arr[] = {1, 2, 5, 3, 2}
Output: No
Explanation:
The given array is not a permutation of numbers from 1 to N, because it contains 2 twice, and 4 is missing for the array to represent a permutation of length 5.

Input: arr[] = {1, 2, 5, 3, 4}
Output: Yes
Explanation:
Given array contains all integers from 1 to 5 exactly once. Hence, it represents a permutation of length 5.

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

Naive Approach: Clearly, the given array will represent a permutation of length N only, where N is the length of the array. So we have to search for each element from 1 to N in the given array. If all the elements are found then the array represents a permutation else it does not.

Time Complexity: O(N2)

Efficient Approach:
The above method can be optimized using a set data structure.

1. Traverse the given array and insert every element in the set data structure.
2. Also, find the maximum element in the array. This maximum element will be value N which will represent the size of the set.
3. After traversal of the array, check if the size of the set is equal to N.
4. If the size of the set if equal to N then the array represents a permutation else it doesn’t.

Below is the implementation of the above approach:

## C++

 `// C++ Program to decide if an ` `// array represents a permutation or not ` ` `  `#include ` `using` `namespace` `std; ` ` `  `// Function to check if an ` `// array represents a permutation or not ` `bool` `permutation(``int` `arr[], ``int` `n) ` `{ ` `    ``// Set to check the count ` `    ``// of non-repeating elements ` `    ``set<``int``> hash; ` ` `  `    ``int` `maxEle = 0; ` ` `  `    ``for` `(``int` `i = 0; i < n; i++) { ` ` `  `        ``// Insert all elements in the set ` `        ``hash.insert(arr[i]); ` ` `  `        ``// Calculating the max element ` `        ``maxEle = max(maxEle, arr[i]); ` `    ``} ` ` `  `    ``if` `(maxEle != n) ` `        ``return` `false``; ` ` `  `    ``// Check if set size is equal to n ` `    ``if` `(hash.size() == n) ` `        ``return` `true``; ` ` `  `    ``return` `false``; ` `} ` ` `  `// Driver code ` `int` `main() ` `{ ` `    ``int` `arr[] = { 1, 2, 5, 3, 2 }; ` `    ``int` `n = ``sizeof``(arr) / ``sizeof``(``int``); ` ` `  `    ``if` `(permutation(arr, n)) ` `        ``cout << ``"Yes"` `<< endl; ` `    ``else` `        ``cout << ``"No"` `<< endl; ` ` `  `    ``return` `0; ` `} `

## Java

 `// Java Program to decide if an ` `// array represents a permutation or not ` `import` `java.util.*; ` ` `  `class` `GFG{ ` ` `  `// Function to check if an ` `// array represents a permutation or not ` `static` `boolean` `permutation(``int` `[]arr, ``int` `n) ` `{ ` `    ``// Set to check the count ` `    ``// of non-repeating elements ` `    ``Set hash = ``new` `HashSet();  ` ` `  `    ``int` `maxEle = ``0``; ` ` `  `    ``for` `(``int` `i = ``0``; i < n; i++) { ` ` `  `        ``// Insert all elements in the set ` `        ``hash.add(arr[i]); ` ` `  `        ``// Calculating the max element ` `        ``maxEle = Math.max(maxEle, arr[i]); ` `    ``} ` ` `  `    ``if` `(maxEle != n) ` `        ``return` `false``; ` ` `  `    ``// Check if set size is equal to n ` `    ``if` `(hash.size() == n) ` `        ``return` `true``; ` ` `  `    ``return` `false``; ` `} ` ` `  `// Driver code ` `public` `static` `void` `main(String args[]) ` `{ ` `    ``int` `arr[] = { ``1``, ``2``, ``5``, ``3``, ``2` `}; ` `    ``int` `n = arr.length; ` ` `  `    ``if` `(permutation(arr, n)) ` `        ``System.out.println(``"Yes"``); ` `    ``else` `        ``System.out.println(``"No"``); ` `} ` `} ` ` `  `// This code is contributed by Surendra_Gangwar `

## Python3

 `# Python3 Program to decide if an ` `# array represents a permutation or not ` ` `  `# Function to check if an ` `# array represents a permutation or not ` `def` `permutation(arr, n): ` `     `  `        ``# Set to check the count ` `    ``# of non-repeating elements ` `    ``s ``=` `set``() ` ` `  `    ``maxEle ``=` `0``; ` ` `  `    ``for` `i ``in` `range``(n): ` `   `  `        ``# Insert all elements in the set ` `        ``s.add(arr[i]); ` ` `  `        ``# Calculating the max element ` `        ``maxEle ``=` `max``(maxEle, arr[i]); ` `     `  `    ``if` `(maxEle !``=` `n): ` `        ``return` `False` ` `  `    ``# Check if set size is equal to n ` `    ``if` `(``len``(s) ``=``=` `n): ` `        ``return` `True``; ` ` `  `    ``return` `False``; ` ` `  `# Driver code ` `if` `__name__``=``=``'__main__'``:  ` ` `  `    ``arr ``=` `[ ``1``, ``2``, ``5``, ``3``, ``2` `] ` `    ``n ``=` `len``(arr) ` ` `  `    ``if` `(permutation(arr, n)): ` `        ``print``(``"Yes"``) ` `    ``else``: ` `        ``print``(``"No"``) ` ` `  `# This code is contributed by Princi Singh `

## C#

 `// C# Program to decide if an ` `// array represents a permutation or not ` `using` `System; ` `using` `System.Collections.Generic; ` ` `  `class` `GFG{ ` `  `  `// Function to check if an ` `// array represents a permutation or not ` `static` `bool` `permutation(``int` `[]arr, ``int` `n) ` `{ ` `    ``// Set to check the count ` `    ``// of non-repeating elements ` `    ``HashSet<``int``> hash = ``new` `HashSet<``int``>();  ` `  `  `    ``int` `maxEle = 0; ` `  `  `    ``for` `(``int` `i = 0; i < n; i++) { ` `  `  `        ``// Insert all elements in the set ` `        ``hash.Add(arr[i]); ` `  `  `        ``// Calculating the max element ` `        ``maxEle = Math.Max(maxEle, arr[i]); ` `    ``} ` `  `  `    ``if` `(maxEle != n) ` `        ``return` `false``; ` `  `  `    ``// Check if set size is equal to n ` `    ``if` `(hash.Count == n) ` `        ``return` `true``; ` `  `  `    ``return` `false``; ` `} ` `  `  `// Driver code ` `public` `static` `void` `Main(String []args) ` `{ ` `    ``int` `[]arr = { 1, 2, 5, 3, 2 }; ` `    ``int` `n = arr.Length; ` `  `  `    ``if` `(permutation(arr, n)) ` `        ``Console.WriteLine(``"Yes"``); ` `    ``else` `        ``Console.WriteLine(``"No"``); ` `} ` `} ` ` `  `// This code is contributed by Princi Singh `

Output:

```No
```

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