Missing element in Permutation.

An array A consisting of N different integers is given. The array contains integers in the range [1..(N + 1)], which means that exactly one element is missing.

Your goal is to find that missing element.

Write a function:

1
2function solution(A) {
3}
4

that, given an array A, returns the value of the missing element.

For example, given array A such that:

A[0] = 2
A[1] = 3
A[2] = 1
A[3] = 5

the function should return 4, as it is the missing element.

Write an efficient algorithm for the following assumptions:

N is an integer within the range [0..100,000];
the elements of A are all distinct;
each element of array A is an integer within the range [1..(N + 1)].

Points to be Taken

1.1

1
2function solution(A) {
3    const b = A.sort()
4    for(let i=0;i<=b.length;i++) {
5        if(b[i+1] !== b[i] + 1) {
6            return b[i] + 1
7        }
8    }
9    return ''
10}
11

1.2

1
2function solution(A) {
3    const n = A.length + 1
4    const sumOfN = Math.floor(n * (n + 1) / 2)
5    const sumOfAll = A.reduce((a, b) => a + b, 0)
6    return sumOfN - sumOfAll
7}
8

Points to be Taken

Array problem expect empty array condition check and single element check.
Arithmetic progression