set is like a fun that if any elem is repeated it will del it so he will ran the loop 2 times not like 10 or something and it can be more than 2 but cuz there is just 2 kine of numbers in the same list it will get 2 and the repeated one will get del

Reversed isn't repeated in a row, they're far apart, so that doesn't count. See this example:

"apple dog apple dog cat" --> 0

That's the concept of recursion.
As you grow, you'll get to use it more often

every element in arr does arr.acount(x)<=1, if the arr is huge(too many elements), it will take long time...

print and return are not the same ;-)