# What's the time complexity of this algorithm? And Why?

I am stuck by analyzing the time complexity of the following algorithm:

def fun (r, k, d, p):
if d > p:
return r
if d = 0 and p = 0:
r <- r + k
return r
if d > 0:
fun (r, k + 1, d - 1, p)
if p > 0:
fun (r, k - 1, d, p - 1)


The root call will be fun (0, 0, n, n), and n is the size of the problem.

I guess that: The recurrence relation is $T(n, n) = T(n-1, n) + T(n, n-1)$, which is equivalent to $T(2n) = 2T(2n-1) \iff T(m) = 2T(m - 1)$, and so $O(2^m) \iff O(4^n)$.

Is my analysis correct (I know it's not very complete and exact)? If it does have serious flaw, please point it out or show me a correct and complete proof on the time complexity of this algorithm.

• xando, I encourage you to edit the question to explain why the standard techniques don't work: explain why they fail. (Cc: @YuvalFilmus) For instance, do you get a recurrence relation that's hard to solve, and if so, what recurrence do you get? – D.W. Oct 4 '15 at 21:35
• In comments with Polyergic, I've realized that the pseudocode is unclear: it's not clear what you mean for the algorithm to do, when both d>0 and p>0. You don't show what the function returns if we reach the 3rd and 4th if statements. Did you mean to have a return statement after each recursive invocation of fun? (did you mean fun (r, k + 1, d - 1, p) to be return fun (r, k + 1, d - 1, p)?) Or did you mean to have a return statement at the very end of the function body? Please edit your pseudocode to clarify and make sure you show what this returns in all possible cases. – D.W. Oct 9 '15 at 23:42
• To say it in another way: suppose d<=p and d>0 and p>0 all hold. What is supposed to happen? Does the algorithm make 2 recursive invocations to the function? Or does it recursively invoke fun(r, k + 1, d - 1, p) and then immediately return, without recursively invoking fun(r, k - 1, d, p - 1)? If I take your pseudocode literally, it appears that it makes 2 recursive invocations and then returns with an undefined return value -- but that seems odd and makes me wonder if there's a typo/bug in the pseudocode. – D.W. Oct 9 '15 at 23:45

The only two arguments relevant to asymptotic analysis are $d$ and $p$. These arguments (virtually) satisfy $d,p \geq 0$ and $d \leq p$ (we need to shuffle the logic in the function slightly to get this). At each point in the execution, you take the current pair $(d,p)$ and then recursively call the function with the pairs $(d-1,p),(d,p-1)$, avoiding pairs which invalidate the constraints stated above.

We can picture the resulting call tree as a path starting at $(0,0)$. Each time you decrease $p$, add a / step. Each time you decrease $d$, add a \ step. The condition $d \leq p$ guarantees that you never go below the X axis. Moreover, you have a "budget" of $n$ of each step. The total number of leaves in this call tree is exactly the Catalan number $\binom{2n}{n}/(n+1) = \Theta(4^n/n^{3/2})$, and this gives us a lower bound on the running time of the function.

To get an upper bound, note that on the way to each leaf we pass through $2n$ nodes, and this gives an upper bound $2n$ larger than the lower bound, i.e., $\Theta(4^n/\sqrt{n})$.

We have a lower bound of $\Omega(4^n/n^{3/2})$ and an upper bound on $O(4^n/\sqrt{n})$. What are the exact asymptotics? They grow like the total number of paths not crossing the X axis which have at most $n$ steps in each direction. Using Bertrand's ballot theorem we can get an exact expression for this: $$\sum_{0 \leq d \leq p \leq n} \frac{p-d+1}{p+1} \binom{p+d}{p}.$$ It thus remains to estimate this sum asymptotically: $$\sum_{0 \leq d \leq p \leq n} \binom{p+d}{p} - \sum_{0 \leq d \leq p \leq n} \frac{d}{p+1} \binom{p+d}{d} = \\ \sum_{0 \leq d \leq p \leq n} \binom{p+d}{p} - \sum_{0 \leq d \leq p \leq n} \binom{p+d}{p+1} = \\ \sum_{p=0}^n \binom{2p+1}{p+1} - \sum_{p=0}^n \binom{2p+1}{p+2} = \\ \sum_{p=0}^n \frac{1}{p+1} \binom{2p+2}{p} = \Theta\left(\sum_{p=0}^n \frac{4^p}{p^{3/2}}\right) = \Theta\left(\frac{4^n}{n^{3/2}}\right).$$

• I really like your geometric approach using these "steps". Is that a common technique? I have not seen it before. – Andreas T Oct 5 '15 at 8:24
• @AndreasT I wouldn't call it a common technique, indeed normally it doesn't apply. Here the combinatorial interpretation is pretty evident, and it leads one to this kind of solution. – Yuval Filmus Oct 5 '15 at 17:45

Case by case:

1. d > p: Constant time
2. d=0 ∧ p=0: Constant time
3. d > 0: Note that d ≯ p, so we have 0 < d ≤ p, and fun recurses on d-1 until d ≯ 0; since p > 0, this is Linear in d + (case 4).
4. p > 0: Note that d ≯ 0, so we have d ≤ 0 ≤ p (with d < p), and fun recurses on p-1 until p ≯ 0; this is Linear in p + (one of case 1, 2, or 5)
5. d ≤ p < 0: Undefined; I'm assuming this is Constant time

Starting with d = p = n > 0 hits case 3, which is followed by case 4. If n is a whole number, the final case is 2, otherwise the final case is 5. The total time for those cases is d+p+1, or 2n+1.

• 2n. I guess you downvoted because I focused on the reasoning? – ShadSterling Oct 8 '15 at 3:41
• Thanks for editing the answer! The puzzle now is that you concluded that the running time is $O(n)$, but Yuval concluded that the running time is exponential in $n$. That's a pretty substantial difference. – D.W. Oct 8 '15 at 16:49
• Hmm, I think I took the pseudocode ifs to be a "do one of these" while @Yuval took them to be "consider each of these in order". The latter is, of course, what ifs (without else) mean in actual code; I'm used to the former in almost any other context than actual code (including in pseudocode, although usage in pseudocode is inconsistent). – ShadSterling Oct 9 '15 at 20:28
• I see what you mean. The lack of return statements in the latter half of the code makes this pretty confusing. – D.W. Oct 9 '15 at 23:37