Reversing a integer value list

Question

I have a list of integer values which can be anywhere between 1 and 4. So, let's say {1,2,4,1,3,2,1,4,4} for instance. I now want to reverse the values in the following way: All entries with ...

• 1 should be converted to 4,
• 2 should be converted to 3,
• 3 should be converted to 2,
• 4 should be converted to 1.

There are numerous ways to do this but I want to take the most efficient approach.

Any thoughts?

Answer 1

for(int i = 0; i < array.Length; i++)
{
    array[i] = 5 - array[i];
}

Answer 2

Implement this function:

f(x) = 5 - x