Finding Nth item of unsorted list without sorting the list

Question

Hey. I have a very large array and I want to find the Nth largest value. Trivially I can sort the array and then take the Nth element but I\'m only interested in one element

Answer 1

Sorting would require O(nlogn) runtime at minimum - There are very efficient selection algorithms which can solve your problem in linear time.

Partition-based selection (sometimes Quick select), which is based on the idea of quicksort (recursive partitioning), is a good solution (see link for pseudocode + Another example).