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Time - Olga #12

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48 changes: 45 additions & 3 deletions lib/heap_sort.rb
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Original file line number Diff line number Diff line change
@@ -1,8 +1,50 @@


# This method uses a heap to sort an array.
# Time Complexity: ?
# Space Complexity: ?
# Time Complexity: O(n log n)
# Space Complexity: O(1)
def heap_sort(list)
Comment on lines +4 to 6
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@CheezItMan CheezItMan Sep 28, 2020

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👍 , Nice work doing this in-place with heapsort. Using the given array as the heap. Well done.

raise NotImplementedError, "Method not implemented yet..."
n = list.size
return list if n < 2

# heap construction - reorganize original array into a maximum heap
# proceed from right to left, starting halfway back through the array
# O(n)
i = (n - 1) / 2
while i >= 0
heap_down(list, i, n)
i -= 1
end

# sort
# remove the largest remaining item from the heap and
# put it into the array position vacated as the heap shrinks
# O(n log n)
while n > 0
list[0], list[n - 1] = list[n - 1], list[0]

n -= 1
heap_down(list, 0, n)

end
list
end


# max heap heap_down method
def heap_down(list, index, n)
while index * 2 + 1 < n
j = index * 2 + 1 # first child

if j < n - 1 && list[j] < list[j + 1] # greater of two children
j += 1
end

if list[index] > list[j] # stop if move will violate heap order
break
end

list[index], list[j] = list[j], list[index]
index = j
end
end
53 changes: 40 additions & 13 deletions lib/min_heap.rb
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Original file line number Diff line number Diff line change
Expand Up @@ -14,18 +14,28 @@ def initialize
end

# This method adds a HeapNode instance to the heap
# Time Complexity: ?
# Space Complexity: ?
# Time Complexity: O(log n) - worst case, when need heap-up, O(1) - best case
# Space Complexity: O(log n) - worst case, when need heap-up, O(1) - best case
def add(key, value = key)
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@CheezItMan CheezItMan Sep 28, 2020

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👍

raise NotImplementedError, "Method not implemented yet..."
new_node = HeapNode.new(key, value)
@store << new_node
heap_up(@store.size-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)
# Space Complexity: O(log n)
def remove()
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👍

raise NotImplementedError, "Method not implemented yet..."
return if @store.empty?
if @store.size == 1
return @store.pop
end
# swap the first and the last element in the underlying array
@store[-1], @store[0] = @store[0], @store[-1]
removed = @store.pop
heap_down(0)
return removed.value
end


Expand All @@ -44,28 +54,45 @@ def to_s
end

# This method returns true if the heap is empty
# Time complexity: ?
# Space complexity: ?
# Time complexity: O(1)
# Space complexity: O(1)
def empty?
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👍

raise NotImplementedError, "Method not implemented yet..."
@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)
# Space complexity: O(log n)
def heap_up(index)
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👍


# base case
parent_index = (index - 1) / 2
return if parent_index < 0 || @store[index].key >= @store[parent_index].key

# swap
@store[index], @store[parent_index] = @store[parent_index], @store[index]
# recursive case
heap_up(parent_index)
end

# This helper method takes an index and
# moves it up the heap if it's smaller
# than it's parent node.
def heap_down(index)
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@CheezItMan CheezItMan Sep 28, 2020

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👍

raise NotImplementedError, "Method not implemented yet..."
n = @store.size
while index * 2 + 1 < n
j = index * 2 + 1

j += 1 if j < n - 1 && @store[j + 1].key < @store[j].key
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Nice work handling the left and right children here.


break if @store[index].key <= @store[j].key

@store[index], @store[j] = @store[j], @store[index]
index = j
end
end

# If you want a swap method... you're welcome
Expand Down
8 changes: 4 additions & 4 deletions test/heapsort_test.rb
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Original file line number Diff line number Diff line change
@@ -1,12 +1,12 @@
require_relative "test_helper"

xdescribe "heapsort" do
describe "heapsort" do
it "sorts an empty array" do
# Arrange
list = []

# Act
result = heapsort(list)
result = heap_sort(list)

# Assert
expect(result).must_equal []
Expand All @@ -17,7 +17,7 @@
list = [5]

# Act
result = heapsort(list)
result = heap_sort(list)

# Assert
expect(result).must_equal [5]
Expand All @@ -28,7 +28,7 @@
list = [5, 27, 3, 16, -50]

# Act
result = heapsort(list)
result = heap_sort(list)

# Assert
expect(result).must_equal [-50, 3, 5, 16, 27]
Expand Down