Given the following code:

int f3(int n)
   if(n <= 2) return 1;
   f3(1 + f3(n-2));
   return n - 1;

I was trying to find the time complexity and I got this expression: $T(n)=1+T(T(n-2))$ which I've never seen before.

I know the answer is $O(2^{\frac{n}{2}})$ but I have no idea how to get there. Can anyone explain the calculation process?


1 Answer 1



The correct recurrence is


That should be easier to solve.

  • $\begingroup$ Thank you for commenting. I was trying to work it from here, but I keep getting the same wrong answer ($O(\frac{n}{2})$). I think that the problem is that I've never seen this type of recursion. If you could give some more explanation it would be much appreciated. $\endgroup$
    – complexity
    Commented Feb 13, 2023 at 17:40
  • $\begingroup$ @complexity: who told you that this answer is wrong ? The solution is virtually in front of your eyes, please read again. $\endgroup$
    – user16034
    Commented Feb 13, 2023 at 17:43
  • $\begingroup$ This problem is taken from a previous test at my university, and the official answer there is $O(2^{\frac{n}{2}})$, and when I try to calculate myself I get $O(\frac{n}{2})=O(n)$. So it is either I'm missing something, or one of the answers is wrong $\endgroup$
    – complexity
    Commented Feb 13, 2023 at 18:11
  • $\begingroup$ @complexity: how to you get this $O$ answer ? $\endgroup$
    – user16034
    Commented Feb 13, 2023 at 19:50

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