Finding value inside array without using history

How can you remember something if you choose not to remember it? The changed value has to be stored somewhere somehow before it can be recovered, even Windows has that recycle bin. For your question, you could have made a copy of the original array, then the original value of a changed element in the original array can be retrieved from the copied array, e.g.

//
// Instantiate the source array.
//
int[] source = new int[5];
source[0] = 1;
source[1] = 2;
source[2] = 3;
source[3] = 4;
source[4] = 5;
//
// Instantiate and allocate the copy array.
//
int[] copy = new int[source.Length];
//
// Copy the source to the copy array.
//
Array.Copy(source, copy, source.Length);

// change one of the source eleement
source[3] = 0;

// Find the original value of the changed element
for(int i=0; i < copy.Length; i++)
{
    if(source[i] != copy[i]) Console.WriteLine(copy[i]);
}

See demo at Array.Copy C#[^]