A zero-indexed 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 a zero-indexed 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.
Assume that:
- 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)].
Complexity:
- expected worst-case time complexity is O(N);
- expected worst-case space complexity is O(1), beyond input storage (not counting the storage required for input arguments).
Elements of input arrays can be modified.