Recurrence for quaternary search algorithm

Question

I have to come up with the recurrence for the quaternary search algorithm.

My initial thought is $T(n)=4T(\frac{n}{4})+c$ because the algorithm examines all four subproblems, and each is one quarter the size of the entire array. But this can't be right because that yields a complexity of $O(n)$. I looked on Google and the complexity of quaternary searches are supposed to be $\log_4n$, but I don't know what the recurrence would be to get me that complexity.

Any help would be greatly appreciated. Thanks!

Answer 1

Your recurrence is wrong, which is why you get the wrong answer. I suspect that the reason your recurrence is wrong is that you don't quite understand the algorithm. Try mimicking your reasoning on binary search until you successfully get the correct running time in that case.

Moreover, as mentioned in the comments, the running time (assuming constant-time comparisons and the RAM machine model) is $\Theta(\log n)$, but cannot get more accurate than that. To get $\log_4 n$ you need to count something more specific, such as the number of comparisons performed by the algorithm.

Answer 2

The correct recursion formula is $T(n) = T(\frac{n}{4}) + c$, which yields $O(lgn)$. Because the base is 4 here, for quaternary searches it becomes $O(log_4n)$.