if $a = [1,2,3,4,6,7]
kthSmallest($a, 4) = 6
Algorithm:
1) If n is small, just sort and return
2) Otherwise, group into 5-element subsets and mind the median
3) Find the median of the medians
4) Find L and U sets
- L is numbers lower than the median of medians
- U is numbers higher than the median of medians
5) Recursive step
- if k is the median of medians, return that
- Otherwise, recusively search in smaller group.