/*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:

function solution($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)].*/
function solution($A) {
    //Gets the length of the Array plus 1
    $N = count($A)+1;
    //Time complexity formula
    $result = intval(floor($N * ($N+1) / 2) - array_sum($A)); //trasnforms tthe result into integer
    
    return $result;
}
