Construct a sample tree with the rebalance implementation missing

notes-and-examples/2026.06.12/overview.md

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+## Outline
+
+- Write a red-black tree with rebalancing
+- Assignment: finish the red-black tree
+- Next week: introduce graphs and graph algorithms

notes-and-examples/2026.06.12/red_black_tree.py

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+class Node:
+    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
+
+class RedBlackTree:
+    def __init__(self):
+        self.root: Node | None = None
+
+    def insert(self, value):
+        # assumption: inserting the same value twice will
+        # insert nothing the second time
+
+        node = Node(value)
+        if self.root is None:
+            self.root = node
+            return
+
+        # insert like a normal binary search tree
+        prev: Node | None = None
+        current = self.root
+        while current:
+            if current.value == value:
+                return
+            prev = current
+            if current.value > value:
+                current = current.left
+            elif current.value < value:
+                current = current.right
+
+        if prev and prev.value > value:
+            prev.left = node
+        elif prev and prev.value < value:
+            prev.right = node
+
+    def delete(self, value, root=None, parent=None):
+        # does nothing if the value doesn't exist
+        if parent is None:
+            root = self.root
+            if root is None:
+                return
+
+        if root.value == value:
+            new_root: Node | None
+            if root.left is None:
+                new_root = root.right
+            elif root.right is None:
+                new_root = root.left
+            else:
+                # both left and right subtrees exist
+                # make left subtree root the tree root, and re-attach
+                # the right subtree
+                right_subtree_root = root.right
+                new_root = root.left
+
+                prev = None
+                current = new_root
+                while current:
+                    prev = current
+                    current = current.right
+                prev.right = right_subtree_root
+
+            if parent is None:
+                self.root = new_root
+            else:
+                if parent.value > root.value:
+                    parent.left = new_root
+                else:
+                    parent.right = new_root
+        elif root.value > value:
+            self.delete(value, root.left, root)
+        elif root.value < value:
+            self.delete(value, root.right, root)
+
+    def rebalance(self):
+        # this is what we'll be implementing
+        pass
+
+    def visualize(self) -> str:
+        # renders the tree top-down with the root on top and branches
+        # ( / and \ ) drawn down to each child. spacing is computed so that
+        # subtrees never overlap, no matter the shape of the tree.
+        if self.root is None:
+            return "<empty tree>"
+
+        # render() returns, for the subtree rooted at `node`:
+        #   lines  - the block of text drawing the subtree
+        #   width  - how many characters wide that block is
+        #   height - how many lines tall that block is
+        #   middle - the column where this node's value is centred (so the
+        #            caller knows where to attach its branch)
+        def render(node: Node) -> tuple[list[str], int, int, int]:
+            label = str(node.value)
+            label_width = len(label)
+
+            # leaf: just the value on a single line
+            if node.left is None and node.right is None:
+                return [label], label_width, 1, label_width // 2
+
+            # only a left child
+            if node.right is None:
+                lines, width, height, mid = render(node.left)
+                first = (mid + 1) * " " + (width - mid - 1) * "_" + label
+                second = mid * " " + "/" + (width - mid - 1 + label_width) * " "
+                shifted = [line + label_width * " " for line in lines]
+                return [first, second] + shifted, width + label_width, height + 2, width + label_width // 2
+
+            # only a right child
+            if node.left is None:
+                lines, width, height, mid = render(node.right)
+                first = label + mid * "_" + (width - mid) * " "
+                second = (label_width + mid) * " " + "\\" + (width - mid - 1) * " "
+                shifted = [label_width * " " + line for line in lines]
+                return [first, second] + shifted, width + label_width, height + 2, label_width // 2
+
+            # two children: render each side, then place this node between them
+            left_lines, left_w, left_h, left_mid = render(node.left)
+            right_lines, right_w, right_h, right_mid = render(node.right)
+            first = (
+                (left_mid + 1) * " "
+                + (left_w - left_mid - 1) * "_"
+                + label
+                + right_mid * "_"
+                + (right_w - right_mid) * " "
+            )
+            second = (
+                left_mid * " "
+                + "/"
+                + (left_w - left_mid - 1 + label_width + right_mid) * " "
+                + "\\"
+                + (right_w - right_mid - 1) * " "
+            )
+            # pad the shorter side so the two blocks line up row-for-row
+            if left_h < right_h:
+                left_lines += [left_w * " "] * (right_h - left_h)
+            elif right_h < left_h:
+                right_lines += [right_w * " "] * (left_h - right_h)
+            merged = [l + label_width * " " + r for l, r in zip(left_lines, right_lines)]
+            return (
+                [first, second] + merged,
+                left_w + right_w + label_width,
+                max(left_h, right_h) + 2,
+                left_w + label_width // 2,
+            )
+
+        lines, _, _, _ = render(self.root)
+        return "\n".join(lines)
+
+# Sample visualizations
+tree = RedBlackTree()
+tree.insert(20)
+tree.insert(10)
+tree.insert(5)
+tree.insert(30)
+tree.insert(40)
+print(tree.visualize())
+
+tree.delete(20)
+print(tree.visualize())
+
+tree.delete(40)
+tree.delete(5)
+print(tree.visualize())
+
+tree.delete(10)
+print(tree.visualize())
+
+tree.delete(30)
+print(tree.visualize())
+
+# attempt duplicate delete
+tree.delete(30)
+print(tree.visualize())
+