diff --git a/scratch/deepmind/part_two/bst-checker.py b/scratch/deepmind/part_two/bst-checker.py
new file mode 100644
index 000000000..fd0374a9c
--- /dev/null
+++ b/scratch/deepmind/part_two/bst-checker.py
@@ -0,0 +1,110 @@
+import unittest
+from collections import deque
+
+
+# While this function solves the problem, it uses O(n) space since we're storing
+# all of the less-thans and greater-thans.
+def is_binary_search_tree_first_attempt(root):
+    q = deque()
+    q.append((set(), set(), root))
+
+    while q:
+        lts, gts, node = q.popleft()
+
+        if not all([node.value < lt for lt in lts]):
+            return False
+        if not all([node.value > gt for gt in gts]):
+            return False
+
+        if node.left:
+            q.append((lts | {node.value}, gts, node.left))
+        if node.right:
+            q.append((lts, gts | {node.value}, node.right))
+
+    return True
+
+
+# While I did not originally solve this problem this way, when I learned that I
+# could condense the space of my solution's runtime, I wrote this.
+def is_binary_search_tree(root):
+    q = deque()
+    q.append((None, None, root))
+
+    while q:
+        lt, gt, node = q.popleft()
+
+        if not lt is None and node.value >= lt:
+            return False
+        if not gt is None and node.value <= gt:
+            return False
+
+        if node.left:
+            q.append((node.value, gt, node.left))
+        if node.right:
+            q.append((lt, node.value, node.right))
+
+    return True
+
+
+# Tests
+class Test(unittest.TestCase):
+    class BinaryTreeNode(object):
+        def __init__(self, value):
+            self.value = value
+            self.left = None
+            self.right = None
+
+        def insert_left(self, value):
+            self.left = Test.BinaryTreeNode(value)
+            return self.left
+
+        def insert_right(self, value):
+            self.right = Test.BinaryTreeNode(value)
+            return self.right
+
+    def test_valid_full_tree(self):
+        tree = Test.BinaryTreeNode(50)
+        left = tree.insert_left(30)
+        right = tree.insert_right(70)
+        left.insert_left(10)
+        left.insert_right(40)
+        right.insert_left(60)
+        right.insert_right(80)
+        result = is_binary_search_tree(tree)
+        self.assertTrue(result)
+
+    def test_both_subtrees_valid(self):
+        tree = Test.BinaryTreeNode(50)
+        left = tree.insert_left(30)
+        right = tree.insert_right(80)
+        left.insert_left(20)
+        left.insert_right(60)
+        right.insert_left(70)
+        right.insert_right(90)
+        result = is_binary_search_tree(tree)
+        self.assertFalse(result)
+
+    def test_descending_linked_list(self):
+        tree = Test.BinaryTreeNode(50)
+        left = tree.insert_left(40)
+        left_left = left.insert_left(30)
+        left_left_left = left_left.insert_left(20)
+        left_left_left.insert_left(10)
+        result = is_binary_search_tree(tree)
+        self.assertTrue(result)
+
+    def test_out_of_order_linked_list(self):
+        tree = Test.BinaryTreeNode(50)
+        right = tree.insert_right(70)
+        right_right = right.insert_right(60)
+        right_right.insert_right(80)
+        result = is_binary_search_tree(tree)
+        self.assertFalse(result)
+
+    def test_one_node_tree(self):
+        tree = Test.BinaryTreeNode(50)
+        result = is_binary_search_tree(tree)
+        self.assertTrue(result)
+
+
+unittest.main(verbosity=2)
diff --git a/scratch/deepmind/part_two/todo.org b/scratch/deepmind/part_two/todo.org
index 3ddd9206a..97b5e6fad 100644
--- a/scratch/deepmind/part_two/todo.org
+++ b/scratch/deepmind/part_two/todo.org
@@ -22,7 +22,7 @@
 ** DONE Merging Meeting Times
 * Trees and graphs
 ** DONE Balanced Binary Tree
-** TODO Binary Search Tree Checker
+** DONE Binary Search Tree Checker
 ** TODO 2nd Largest Item in a Binary Search Tree
 ** TODO Graph Coloring
 ** TODO MeshMessage
@@ -33,7 +33,7 @@
 ** TODO Making Change
 ** TODO The Cake Thief
 ** DONE Balanced Binary Tree
-** TODO Binary Search Tree Checker
+** DONE Binary Search Tree Checker
 ** TODO 2nd Largest Item in a Binary Search Tree
 * Queues and stacks
 ** TODO Largest Stack