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:

1function solution(A) {2}

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

1function solution(A) {2 const b = A.sort()3 for(let i=0;i<=b.length;i++) {4 if(b[i+1] !== b[i] + 1) {5 return b[i] + 16 }7 }8 return ''9}

1.2

1function solution(A) {2 const n = A.length + 13 const sumOfN = Math.floor(n * (n + 1) / 2)4 const sumOfAll = A.reduce((a, b) => a + b, 0)5 return sumOfN - sumOfAll6}

## Points to be Taken

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