Codility #9-PermCheck

function solution(A) {

let numbers = {};

let n = A.length;

let sumAll = 0;

let sumN = n * (n + 1) / 2; // sum(1~N):

for (let i = 0; i < n; i++) {

if (numbers[A[i]]) {

return 0; // duplicated

} else {

numbers[A[i]] = true;

}

sumAll += A[i];

}

return sumAll === sumN ? 1 : 0;

}