 # 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:

`1function solution(A) {2}`

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

For example, given array A such that:

• A = 2
• A = 3
• A = 1
• A = 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

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