Add recursive binary search for ruby (ch. 1) and improve recursive max for ruby (ch. 4) (#113)

* add recursive binary search for ruby

* account for cases with empty lists and lists with only one item

* use p instead of print to show nil value

* add newline at end of file

01_introduction_to_algorithms/ruby/02_recursive_binary_search.rb

@@ -0,0 +1,26 @@
+def binary_search(list, item, low = 0, high = list.length - 1)
+  while low <= high
+    mid = (low + high) / 2
+    guess = list[mid]
+
+    if guess == item
+      return mid
+    elsif guess > item
+      high = mid - 1
+      binary_search(list, item, low, high)
+    else
+      low = mid + 1
+      binary_search(list, item, low, high)
+    end
+  end
+
+  # If item is not found
+  return nil
+end
+
+# Create array with numbers 1 through 100
+my_list = (1..100).to_a
+
+puts binary_search(my_list, 2) # => 1
+puts binary_search(my_list, 50) # => 49
+p binary_search(my_list, 101) # => nil

04_quicksort/ruby/04_recursive_max.rb

@@ -1,7 +1,8 @@
 def max(list)
-  if list.length == 2
+  if list.empty?
-    # condition ? then : else
+    nil
-    list[0] > list[1] ? list[0] : list[1]
+  elsif list.length == 1
+    numbers[0]
   else
     sub_max = max(list[1..-1])
     list[0] > sub_max ? list[0] : sub_max