"""
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)].
"""

def solution(A):
    last = len(A) + 1

    S = set(A)

    if last == 1 or last not in S:
        return last

    for x in range(1, last):
        if x not in S:
            return x