Problem 9.1

Largest i numbers in sorted order

Given a set of $n$ numbers, we wish to find the $i$ largest in sorted order using a comparison based algorithm. Find the algorithm that implements each of the following methods with the best asymptotic worst-case running time, and analyze the running time of the algorithms in terms of $n$ and $i$.

  1. Sort the numbers, and list the $i$ largest
  2. Build a max-priority queue from the numbers, and call `EXTRACT-MAX` $i$ times
  3. Use an order-statistic algorithm to find the $i$th largest number, partition around that number, and sort the $i$ largest numbers.


We can sort with any of the $n\lg{n}$ algorithms, that is, merge sort or heap sort and then just take the first $i$ elements linearly.

This will take $n\lg{n} + i$ time.

Max-priority queue

We can build the heap linearly and then take each of the largest $i$ elements in logarithmic time.

This takes $n + i\lg{n}$.

Partition and sort

Let's assume we use the SELECT algorithm from the chapter. We can find the $i$th order statistic and partition around it in $n$ time and then we need to do a sort in $i\lg{i}$.

This takes $n + i\lg{i}$