MissingElement.js

function solution(A) {
    // write your code in JavaScript (Node.js 8.9.4)
    let n = A.length;
    let totalSum = ((n+1)*(n+2))/2;
    let sum = 0;
    A.forEach(element => {
        sum += element;
    });
    return (totalSum - sum);
}
console.log(solution([2,3,1,5]));



// 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:

//     class Solution { public int solution(int[] A); }

// 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)].