# [Algorithm] PermCheck

## Question

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].

## Solution (TypeScript)

```
function solution(a: number[]) {
const array = [];
const length = a.length;
for (let i = 0; i < length; i += 1) {
const val = a[i];
if (array[val - 1] || val > length) {
return 0;
}
array[val - 1] = val;
}
return 1;
}
```

Author: Codility, Lesson 4

20200107 Charyum.Park