Add quicksort algorithm

quicksort.rb

@@ -0,0 +1,15 @@
+def quicksort
+  return [] if empty?
+
+  # chose a random pivot value
+  pivot = delete_at(rand(size))
+  # partition array into 2 arrays and comparing them to each other and eventually returning
+  # array with the pivot value sorted
+  left, right = partition(&pivot.method(:>))
+
+  # recursively calling the quicksort method on itself
+  return *left.quicksort, pivot, *right.quicksort
+end
+
+arr = [34, 2, 1, 5, 3]
+p arr.quicksort