Shubha 👾

lib/heap_sort.rb

@@ -1,8 +1,61 @@
+require "pry"
+# This method uses a heap to sort an array.
+# Time Complexity:  O (n log n) where n is the number of elements in the heap
+# Space Complexity: O (1)
 
+# Adapted from diagrams and pseudocode in https://medium.com/basecs/heapify-all-the-things-with-heap-sort-55ee1c93af82
 
-# This method uses a heap to sort an array.
-# Time Complexity:  ?
-# Space Complexity: ?
-def heap_sort(list)
-  raise NotImplementedError, "Method not implemented yet..."
+def heapsort(list)
+  limit = list.length
+  build_max_heap(list)
+
+  while limit > 0 # O(n)
+    swap(list, 0, limit - 1)
+    limit -= 1
+    heapify(list, 0, limit) # O(log n)
+  end
+
+  return list
+end
+
+def build_max_heap(list)
+  index = list.length/2 - 1
+  while index >= 0
+    heapify(list, index, list.length) 
+    index -= 1
+  end
+end
+
+
+def heapify(list, index, limit)
+  root = index
+
+  while index < limit
+    left = (2 * index) + 1
+    right = (2 * index) + 2
+
+    if left >= limit && right >= limit
+      return
+    elsif right >= limit
+      max = left
+    else
+      max = [left, right].max_by {|x| list[x]}
+    end
+
+    if list[max] > list[root]
+      root = max
+    else
+      return
+    end
+
+    swap(list, root, index)
+
+    index = root
+  end
+end
+
+def swap(list, index_1, index_2)
+  temp = list[index_1]
+  list[index_1] = list[index_2]
+  list[index_2] = temp
 end

lib/min_heap.rb

@@ -14,18 +14,23 @@ def initialize
   end
 
   # This method adds a HeapNode instance to the heap
-  # Time Complexity: ?
-  # Space Complexity: ?
+  # Time Complexity: O (log n) where n is the number of nodes in the store
+  # Space Complexity: O(1)
   def add(key, value = key)
-    raise NotImplementedError, "Method not implemented yet..."
+    new_node = HeapNode.new(key, value)
+    @store << new_node
+    heap_up(@store.length - 1)
   end
 
   # This method removes and returns an element from the heap
   #   maintaining the heap structure
-  # Time Complexity: ?
-  # Space Complexity: ?
+  # Time Complexity: O (log n) where n is the number of nodes in the store
+  # Space Complexity: O(1) 
   def remove()
-    raise NotImplementedError, "Method not implemented yet..."
+    swap(0, @store.length - 1)
+    removed_node = @store.pop
+    heap_down(0)
+    return removed_node.value
   end
 
 
@@ -47,25 +52,43 @@ def to_s
   # Time complexity: ?
   # Space complexity: ?
   def empty?
-    raise NotImplementedError, "Method not implemented yet..."
+    return @store.empty?
   end
 
   private
 
   # This helper method takes an index and
   #  moves it up the heap, if it is less than it's parent node.
   #  It could be **very** helpful for the add method.
-  # Time complexity: ?
-  # Space complexity: ?
+  # Time complexity: O(log n) where n is the number of nodes in the store
+  # Space complexity: O(1)
   def heap_up(index)
-
+    return if index == 0
+    parent_index = (index - 1) / 2
+    if @store[index].key < @store[parent_index].key
+      swap(index, parent_index)
+      heap_up(parent_index)
+    else
+      return
+    end
   end
 
   # This helper method takes an index and 
-  #  moves it up the heap if it's smaller
-  #  than it's parent node.
+  #  moves it down the heap 
+  # Time complexity: O (log n) where n is the number of nodes in the store
+  # Space complexity: O(1)
   def heap_down(index)
-    raise NotImplementedError, "Method not implemented yet..."
+    left = (2 * index) + 1
+    right = (2 * index) + 2
+    if !@store[left] && !@store[right]
+      return 
+    elsif !@store[right]
+      min = left
+    else
+      min = [left, right].min_by {|x| @store[x].key}
+    end
+    swap(index, min)
+    heap_down(min)
   end
 
   # If you want a swap method... you're welcome

test/heapsort_test.rb

@@ -1,6 +1,6 @@
 require_relative "test_helper"
 
-xdescribe "heapsort" do
+describe "heapsort" do
   it "sorts an empty array" do
     # Arrange 
     list = []