Finding missing elements in an array

Question

Given you have an array A[1..n] of size n, it contains elements from the set {1..n}. However, two of the elements are missing, (and perhaps two of the array elements are rep

Answer 1

Cycle each element 0...n-1.

x = abs(A[i]) (with i = 0...n-1);

A[x - 1] can be: 
> 0: we haven't checked the element, change its sign to negative:
    A[x - 1] = -A[x - 1]
< 0: we already found the same number

At the end of the cycle, pass each A[0...n-1]. The index of the positive elements + 1 is the missing numbers.

So if

y = abs(A[i]) > 0: i + 1 is missing.

In C#

var arr = new[] { 1, 2, 1, 2, 4 };

for (int i = 0; i < arr.Length; i++) {
    int x = Math.Abs(arr[i]);
    int y = arr[x - 1];
    if (y > 0) {
        arr[x - 1] = -arr[x - 1];
    }
}

for (int i = 0; i < arr.Length; i++) {
    int x = arr[i];
    if (x > 0) {
        Console.WriteLine("Missing {0}", i + 1);
    } else {
        arr[i] = -arr[i];
    }
}

And the array is as good as new.