BST and Mergesort Code for Sept 14th

BST.rb

@@ -0,0 +1,65 @@
+# pg 398 of Algo 4th Edition
+
+
+class BST
+  def initialize
+    @root = nil
+  end
+
+  class Node
+    attr_reader :key
+    attr_accessor :value, :left, :right, :node_count
+
+    def initialize(key, value, node_count)
+      @key = key
+      @value = value
+      @left = nil
+      @right = nil
+      @node_count = node_count
+    end
+  end
+
+  def tree_size
+    size(@root)
+  end
+
+  def size(node)
+    return 0 unless node
+    node.node_count
+  end
+
+  def get(key)
+    get_helper(@root, key)
+  end
+
+  def put(key, value)
+    @root = put_helper(@root, key, value)
+  end
+
+  private
+
+  def get_helper(node, key)
+    return nil unless node
+    if key < node.key
+      get_helper(node.left, key)
+    elsif key > node.key
+      get_helper(node.right, key)
+    else
+      node.value
+    end
+  end
+
+  def put_helper(node, key, value)
+    return Node.new(key, value, 1) unless node
+    if key < node.key
+      node.left = put_helper(node.left, key, value)
+    elsif key > node.key
+      node.right = put_helper(node.right, key, value)
+    else
+      node.value = value
+    end
+    node.node_count = size(node.left) + size(node.right) + 1;
+    p "Hello"
+    return node
+  end
+end

MergeSort.rb

@@ -0,0 +1,55 @@
+# pg 271 of Algo 4th Edition
+
+def merge!(arr, low, mid, high, aux_arr=[])
+  (0...arr.length).each do |i| # Copies the arr into the aux_arr
+    aux_arr[i] = arr[i]
+  end
+
+  i = low
+  j = mid + 1
+
+  (low..high).each do |k|
+    if (i > mid)
+      arr[k] = aux_arr[j]
+      j += 1 # Wish there was a clean way to do j++/i++
+    elsif (j > high)
+      arr[k] = aux_arr[i]
+      i += 1
+    elsif (aux_arr[j] < aux_arr[i])
+      arr[k] = aux_arr[j]
+      j += 1
+    else
+      arr[k] = aux_arr[i]
+      i += 1
+    end
+  end
+  # p aux_arr.object_id # Shows that no new aux_arrs are created
+  arr # Line needed for base case, won't work for <=2 element arrays without it
+end
+
+# pg 273 of Algo 4th Edition
+
+def mergesort!(arr, low=0, high = arr.length-1, aux_arr=arr.dup) # Did not know about the arr.length as a param
+  # aux_arr = arr.dup # Don't need this line if I create the aux_arr in mergesort! params
+  # p aux_arr.object_id # Shows that no new aux_arrs are created
+
+  mid = low + (high - low)/2
+
+  return merge!(arr,low,mid,high, aux_arr) if low+1 >= high # Base case
+
+  mergesort!(arr, low, mid, aux_arr)
+  mergesort!(arr, mid+1, high, aux_arr)
+  merge!(arr,low, mid, high, aux_arr)
+  arr
+end
+
+
+1000.times do |x|
+  arr = []
+  rand(300).times do |y|
+    arr << rand
+  end
+  if (arr.sort != mergesort!(arr))
+    puts "Problem!"
+  end
+end