### PermCheck

February 07, 2020

A non-empty array A consisting of N integers is given.

A **permutation** is a sequence containing each element **from 1 to N once**, and only once.

For example, array A such that:

A[0] = 4

A[1] = 1

A[2] = 3

A[3] = 2

is a permutation, but array A such that:

A[0] = 4

A[1] = 1

A[2] = 3

is not a permutation, because value 2 is missing.
The goal is to check whether array A is a permutation.

**Write a function:**

function solution(A);

that, given an array A, **returns 1 if array A is a permutation and 0 if it is not**.

For example, given array A such that:

A[0] = 4

A[1] = 1

A[2] = 3

A[3] = 2

the function should return 1.

Given array A such that:

A[0] = 4

A[1] = 1

A[2] = 3

the function should return 0.

Write an efficient algorithm for the following assumptions:

• N is an integer within the range [1..100,000];

• each element of array A is an integer within the range [1..1,000,000,000].

```
function solution(A) {
// write your code in JavaScript (Node.js 8.9.4)
A.sort((a,b)=>a-b)
if(A[0] !== 1) return 0
for(let i=1; i<A.length; i++){
if( A[i] !== A[i-1]+1) return 0
}
return 1
}
```