diff --git a/binary_tree_230809/main_test.go b/binary_tree_230809/main_test.go
new file mode 100644
index 0000000..ed5054c
--- /dev/null
+++ b/binary_tree_230809/main_test.go
@@ -0,0 +1,148 @@
+package main
+
+import (
+	"fmt"
+	"testing"
+)
+
+type node struct {
+	left  *node
+	right *node
+	value string
+}
+
+func preOrder(n *node) {
+	if n == nil {
+		return
+	}
+	visit(n)
+	preOrder(n.left)
+	preOrder(n.right)
+}
+
+func preOrder2(n *node) {
+	stack := MakeStack()
+	p := n
+	for p != nil || !stack.IsEmpty() {
+		if p != nil {
+			visit(p)
+			stack.Push(p)
+			p = p.left
+		} else {
+			p = stack.Pop().(*node)
+			p = p.right
+		}
+	}
+}
+
+func inOrder(n *node) {
+	if n == nil {
+		return
+	}
+	inOrder(n.left)
+	visit(n)
+	inOrder(n.right)
+}
+
+func inOrder2(n *node) {
+	stack := MakeStack()
+	p := n
+	for p != nil || !stack.IsEmpty() {
+		if p != nil {
+			stack.Push(p)
+			p = p.left
+		} else {
+			p = stack.Pop().(*node)
+			visit(p)
+			p = p.right
+		}
+	}
+}
+
+func postOrder(n *node) {
+	if n == nil {
+		return
+	}
+	postOrder(n.left)
+	postOrder(n.right)
+	visit(n)
+}
+
+func postOrder2(n *node) {
+	stack := MakeStack()
+	p := n
+	var r *node = nil
+	for p != nil || !stack.IsEmpty() {
+		if p != nil {
+			stack.Push(p)
+			p = p.left
+		} else {
+			p = stack.Top().(*node)
+			if p.right != nil && p.right != r {
+				p = p.right
+				stack.Push(p)
+				p = p.left
+			} else {
+				p = stack.Pop().(*node)
+				visit(p)
+				r = p
+				p = nil
+			}
+		}
+	}
+}
+
+func TestPreOrder(t *testing.T) {
+	preOrder(makeBinary())
+}
+
+func TestPreOrder2(t *testing.T) {
+	preOrder2(makeBinary())
+}
+
+func TestInOrder(t *testing.T) {
+	inOrder(makeBinary())
+}
+
+func TestInOrder2(t *testing.T) {
+	inOrder2(makeBinary())
+}
+
+func TestPostOrder(t *testing.T) {
+	postOrder(makeBinary())
+}
+
+func TestPostOrder2(t *testing.T) {
+	postOrder2(makeBinary())
+}
+
+func visit(n *node) {
+	if n == nil {
+		return
+	}
+	fmt.Printf("visiting node %s\n", n.value)
+}
+
+func makeBinary() *node {
+	return &node{
+		left: &node{
+			left: &node{
+				value: "D",
+			},
+			right: &node{
+				value: "E",
+			},
+			value: "B",
+		},
+		right: &node{
+			left: &node{
+				value: "F",
+			},
+			right: &node{
+				value: "G",
+			},
+			value: "C",
+		},
+		value: "A",
+	}
+}
diff --git a/binary_tree_230809/stack.go b/binary_tree_230809/stack.go
new file mode 100644
index 0000000..9e3414a
--- /dev/null
+++ b/binary_tree_230809/stack.go
@@ -0,0 +1,35 @@
+package main
+
+type Stack []interface{}
+
+func MakeStack() *Stack {
+	return &Stack{}
+}
+
+func (s Stack) Len() int {
+	return len(s)
+}
+
+func (s Stack) IsEmpty() bool {
+	return len(s) == 0
+}
+
+func (s Stack) Top() interface{} {
+	if s.IsEmpty() {
+		return nil
+	}
+	return s[len(s)-1]
+}
+
+func (s *Stack) Push(item interface{}) {
+	*s = append(*s, item)
+}
+
+func (s *Stack) Pop() interface{} {
+	if s.IsEmpty() {
+		return nil
+	}
+	ret := (*s)[len(*s)-1]
+	*s = (*s)[:len(*s)-1]
+	return ret
+}
diff --git a/prime_factors_230809/main_test.go b/prime_factors_230809/main_test.go
new file mode 100644
index 0000000..cef9158
--- /dev/null
+++ b/prime_factors_230809/main_test.go
@@ -0,0 +1,47 @@
+package main
+
+import (
+	"reflect"
+	"testing"
+)
+
+func PrimeFactorsOf(n int) []int {
+	ret := make([]int, 0)
+	d := 2
+	for d < n {
+		for n%d == 0 {
+			ret = append(ret, d)
+			n /= d
+		}
+		d++
+	}
+	if n > 1 {
+		ret = append(ret, n)
+	}
+	return ret
+}
+
+func TestPrimeFactorsOf(t *testing.T) {
+	tt := []struct {
+		name  string
+		input int
+		want  []int
+	}{
+		{"1", 1, []int{}},
+		{"2", 2, []int{2}},
+		{"3", 3, []int{3}},
+		{"4", 4, []int{2, 2}},
+		{"8", 8, []int{2, 2, 2}},
+		{"9", 9, []int{3, 3}},
+		{"a very large number", 2 * 3 * 17 * 71 * 73 * 101, []int{2, 3, 17, 71, 73, 101}},
+	}
+
+	for _, tc := range tt {
+		t.Run(tc.name, func(t *testing.T) {
+			got := PrimeFactorsOf(tc.input)
+			if !reflect.DeepEqual(got, tc.want) {
+				t.Errorf("PrimeFactorsOf(%d) = %v, want %v\n", tc.input, got, tc.want)
+			}
+		})
+	}
+}
diff --git a/reverse_string_230809/main_test.go b/reverse_string_230809/main_test.go
new file mode 100644
index 0000000..210c85f
--- /dev/null
+++ b/reverse_string_230809/main_test.go
@@ -0,0 +1,32 @@
+package main
+
+import "testing"
+
+func ReverseString(s string) string {
+	ret := []rune(s)
+	for l, r := 0, len(ret)-1; l < r; l, r = l+1, r-1 {
+		ret[l], ret[r] = ret[r], ret[l]
+	}
+	return string(ret)
+}
+
+func TestReverseString(t *testing.T) {
+	tt := []struct {
+		name  string
+		input string
+		want  string
+	}{
+		{"test", "test", "tset"},
+		{"testing", "testing", "gnitset"},
+		{"i am testing", "i am testing", "gnitset ma i"},
+	}
+
+	for _, tc := range tt {
+		t.Run(tc.name, func(t *testing.T) {
+			got := ReverseString(tc.input)
+			if got != tc.want {
+				t.Errorf("ReverseString(%s) = %s, want %s\n", tc.input, got, tc.want)
+			}
+		})
+	}
+}