PermCheck.md

A non-empty zero-indexed 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 a zero-indexed 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.

Assume that:

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]. Complexity:

expected worst-case time complexity is O(N); expected worst-case space complexity is O(N), beyond input storage (not counting the storage required for input arguments). Elements of input arrays can be modified.

function solution(A) {
    // write your code in JavaScript (Node.js 6.4.0)
    const length = A.length;
    
    var result = A.reduce((result, value) => {
        result[value] = true;
        return result;
    }, {})

    for(var i=1; i<=length; i++){
        if(!result[i]){
            return 0;	
        }
    }

    return 1;
}