Add homework for 2026.05.27

notes-and-examples/2026.05.27/homework.py

@@ -42,20 +42,55 @@
 
 
 class Node:
-    def __init__(self, value, left=None, right=None):
+    def __init__(self, value, left=None, right=None, parent=None):
         self.value = value
         self.left: Node | None = left
         self.right: Node | None = right
 
+        # required to re-set the child later
+        self.parent: Node | None = parent
+
 
 def rotate_left(node: Node) -> Node:
-    # TODO
+    if node.right is None:
-    return Node(0)
+        return node
+
+    original_parent = node.parent
+    new_parent = node.right
+    new_right_child = node.right.left
+
+    node.right = new_right_child
+    node.parent = new_parent
+    new_parent.left = node
+
+    if original_parent is not None:
+        if original_parent.left == node:
+            original_parent.left = new_parent
+        elif original_parent.right == node:
+            original_parent.right = new_parent
+
+    return new_parent
 
 
 def rotate_right(node: Node) -> Node:
-    # TODO
+    if node.left is None:
-    return Node(0)
+        return node
+
+    original_parent = node.parent
+    new_parent = node.left
+    new_left_child = node.left.right
+
+    node.left = new_left_child
+    node.parent = new_parent
+    new_parent.right = node
+
+    if original_parent is not None:
+        if original_parent.left == node:
+            original_parent.left = new_parent
+        elif original_parent.right == node:
+            original_parent.right = new_parent
+
+    return new_parent
 
 
 # --- Tests ---------------------------------------------------------------
@@ -67,7 +102,12 @@ def to_tuple(node: Node | None):
     """Turn a tree into nested (value, left, right) tuples for easy comparison."""
     if node is None:
         return None
-    return (node.value, to_tuple(node.left), to_tuple(node.right))
+    return (
+        node.value,
+        to_tuple(node.left),
+        to_tuple(node.right),
+        node.parent.value if node.parent is not None else None,
+    )
 
 
 def trees_equal(a: Node | None, b: Node | None) -> bool:
@@ -110,12 +150,20 @@ if __name__ == "__main__":
         expected = Node(1, left=Node(0), right=Node(3, left=Node(2), right=Node(4)))
         return rotate_right(rotate_left(original)), expected
 
+    def test_rotate_with_parent():
+        # run it on 33 instead of the root
+        root = Node(16, right=Node(33, left=Node(22), right=Node(55)))
+        original = root.right
+        expected = Node(55, left=Node(33, left=Node(22)))
+        return rotate_left(original), expected  # pyright: ignore[reportArgumentType]
+
     tests = [
         ("rotate_right on worksheet example (10)", test_rotate_right_example),
         ("rotate_left on worksheet example (16)", test_rotate_left_example),
         ("rotate_right on two nodes", test_rotate_right_two_nodes),
         ("rotate_left on two nodes", test_rotate_left_two_nodes),
         ("rotate_left then rotate_right restores the tree", test_round_trip),
+        ("rotate_left works when there is a parent", test_rotate_with_parent),
     ]
 
     passed = 0