There are several different selection algorithms, from the much simpler quickselect (expected O(n), worst-case O(n2)) to the more complex median-of-medians algorithm (Θ(n)). Both of these algorithms work by using a quicksort partitioning step (time O(n)) to rearrange the elements and position one element into its proper position. If that element is at the index in question, we're done and can just return that element. Otherwise, we determine which side to recurse on and recurse there.
Let's now make a very strong assumption - suppose that we're using quickselect (pick the pivot randomly) and on each iteration we manage to guess the exact middle of the array. In that case, our algorithm will work like this: we do a partition step, throw away half of the array, then recursively process one half of the array. This means that on each recursive call we end up doing work proportional to the length of the array at that level, but that length keeps decreasing by a factor of two on each iteration. If we work out the math (ignoring constant factors, etc.) we end up getting the following time:
- Work at the first level: n
- Work after one recursive call: n / 2
- Work after two recursive calls: n / 4
- Work after three recursive calls: n / 8
- ...
This means that the total work done is given by
n + n / 2 + n / 4 + n / 8 + n / 16 + ... = n (1 + 1/2 + 1/4 + 1/8 + ...)
Notice that this last term is n times the sum of 1, 1/2, 1/4, 1/8, etc. If you work out this infinite sum, despite the fact that there are infinitely many terms, the total sum is exactly 2. This means that the total work is
n + n / 2 + n / 4 + n / 8 + n / 16 + ... = n (1 + 1/2 + 1/4 + 1/8 + ...) = 2n
This may seem weird, but the idea is that if we do linear work on each level but keep cutting the array in half, we end up doing only roughly 2n work.
An important detail here is that there are indeed O(log n) different iterations here, but not all of them are doing an equal amount of work. Indeed, each iteration does half as much work as the previous iteration. If we ignore the fact that the work is decreasing, you can conclude that the work is O(n log n), which is correct but not a tight bound. This more precise analysis, which uses the fact that the work done keeps decreasing on each iteration, gives the O(n) runtime.
Of course, this is a very optimistic assumption - we almost never get a 50/50 split! - but using a more powerful version of this analysis, you can say that if you can guarantee any constant factor split, the total work done is only some constant multiple of n. If we pick a totally random element on each iteration (as we do in quickselect), then on expectation we only need to pick two elements before we end up picking some pivot element in the middle 50% of the array, which means that, on expectation, only two rounds of picking a pivot are required before we end up picking something that gives a 25/75 split. This is where the expected runtime of O(n) for quickselect comes from.
A formal analysis of the median-of-medians algorithm is much harder because the recurrence is difficult and not easy to analyze. Intuitively, the algorithm works by doing a small amount of work to guarantee a good pivot is chosen. However, because there are two different recursive calls made, an analysis like the above won't work correctly. You can either use an advanced result called the Akra-Bazzi theorem, or use the formal definition of big-O to explicitly prove that the runtime is O(n). For a more detailed analysis, check out "Introduction to Algorithms, Third Edition" by Cormen, Leisserson, Rivest, and Stein.
Hope this helps!
