Task description
<code>
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:

int solution(int A[], int N);

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.

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) (not counting the storage required for input arguments).
</code>

Solution
<code>
int solution(int A[], int N) {
    // write your code in C99 (gcc 6.2.0)
    int i, sum, total;
    
    sum = 0;
    total = 0;
    
    if(N == 0)
        return 1;
        
    for(i = 0; i < N ; i++) {
        sum += A[i];
        total += i+1;
    }
    total += (i+1);
    
    if(sum == total) {
        return (N + 1);
    }
  
    return total > sum ? total - sum: sum - total;
  
}
</code>