From b5085c9f864b4403a24006cd8ade8bcdef8dc69a Mon Sep 17 00:00:00 2001
From: Paul Buetow <git@mx.buetow.org>
Date: Tue, 10 Nov 2020 21:43:56 +0000
Subject: initial balanced bst

---
 set/balancedbst.go | 143 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 143 insertions(+)
 create mode 100644 set/balancedbst.go

diff --git a/set/balancedbst.go b/set/balancedbst.go
new file mode 100644
index 0000000..92fe06e
--- /dev/null
+++ b/set/balancedbst.go
@@ -0,0 +1,143 @@
+package set
+
+type node struct {
+	key   int
+	val   int
+	left  *node
+	right *node
+}
+
+type BalancedBST struct {
+	root *node
+}
+
+func NewBalancedBST() *BalancedBST {
+	return &BalancedBST{}
+}
+
+func (t *BalancedBST) Empty() bool {
+	return t.root == nil
+}
+
+func (t *BalancedBST) Set(key, val int) {
+	if t.Empty() {
+		t.root = &node{key, val, nil, nil}
+		return
+	}
+	ptr, _, err := t.search(&t.root, key)
+	switch err {
+	case nil:
+		// key already in the tree
+		return
+	case NotFound:
+		*ptr = &node{key, val, nil, nil}
+		return
+	default:
+		panic(err)
+	}
+}
+
+func (t *BalancedBST) Get(key int) (int, error) {
+	_, n, err := t.search(&t.root, key)
+	return n.val, err
+}
+
+func (t *BalancedBST) Del(key int) (int, error) {
+	ptr, n, err := t.search(&t.root, key)
+	if err != nil {
+		return 0, err
+	}
+
+	// Case 1: n is leaf node
+	// Case 2: n has one child
+	// Case 3: n has two childs
+
+	switch {
+	case n.left == nil:
+		if n.right == nil {
+			// I am a leaf node
+			*ptr = nil
+			return n.val, nil
+		}
+		// I have a right child
+		*ptr = n.right
+		return n.val, nil
+
+	case n.right == nil:
+		// I have a left child
+		*ptr = n.left
+		return n.val, nil
+	default:
+		// I have two children!
+
+		o, err := t.deleteMin(&n.right)
+		if err != nil {
+			return 0, err
+		}
+
+		o.left = n.left
+		o.right = n.right
+		*ptr = o
+
+		return n.val, nil
+	}
+}
+
+func (t *BalancedBST) search(ptr **node, key int) (**node, *node, error) {
+	n := *ptr
+	if n == nil {
+		return ptr, nil, NotFound
+	}
+
+	switch {
+	case key < n.key:
+		return t.search(&n.left, key)
+	case n.key < key:
+		return t.search(&n.right, key)
+	default:
+		return ptr, n, nil
+	}
+}
+
+func (t *BalancedBST) deleteMin(ptr **node) (*node, error) {
+	ptr, n, err := t.min(ptr)
+	if err != nil {
+		return nil, err
+	}
+
+	*ptr = n.right
+	n.right = nil
+	return n, nil
+}
+
+func (t *BalancedBST) min(ptr **node) (**node, *node, error) {
+	n := *ptr
+	if n == nil {
+		return nil, nil, NotFound
+	}
+
+	for {
+		if n.left == nil {
+			return ptr, n, nil
+		}
+		ptr = &n.left
+		n = *ptr
+	}
+}
+
+/*
+func (t *BalancedBST) max(ptr **node) (**node, *node, error) {
+	n := *ptr
+	if n == nil {
+		return nil, nil, NotFound
+	}
+
+	for {
+		if n.right == nil {
+			return ptr, n, nil
+		}
+		ptr = &n.right
+		n = *ptr
+	}
+}
+*/
-- 
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