diff --git a/README.md b/README.md
index fc4f1e4..56aaefc 100644
--- a/README.md
+++ b/README.md
@@ -98,3 +98,4 @@ problems from
 * [Day 86](https://github.com/vaskoz/dailycodingproblem-go/issues/181)
 * [Day 87](https://github.com/vaskoz/dailycodingproblem-go/issues/183)
 * [Day 88](https://github.com/vaskoz/dailycodingproblem-go/issues/185)
+* [Day 89](https://github.com/vaskoz/dailycodingproblem-go/issues/187)
diff --git a/day89/problem.go b/day89/problem.go
new file mode 100644
index 0000000..1d36eb5
--- /dev/null
+++ b/day89/problem.go
@@ -0,0 +1,24 @@
+package day89
+
+// BST represents a binary search tree.
+type BST struct {
+	Value       int
+	Left, Right *BST
+}
+
+// IsValidBST checks if the given tree conforms to the BST property.
+func IsValidBST(root *BST) bool {
+	const MaxInt = int(^uint(0) >> 1)
+	const MinInt = -MaxInt - 1
+	return isValidBST(root.Left, MinInt, root.Value) && isValidBST(root.Right, root.Value, MaxInt)
+}
+
+func isValidBST(root *BST, min, max int) bool {
+	if root == nil {
+		return true
+	}
+	if root.Value > min && root.Value <= max {
+		return isValidBST(root.Left, min, root.Value) && isValidBST(root.Right, root.Value, max)
+	}
+	return false
+}
diff --git a/day89/problem_test.go b/day89/problem_test.go
new file mode 100644
index 0000000..6cf01c8
--- /dev/null
+++ b/day89/problem_test.go
@@ -0,0 +1,29 @@
+package day89
+
+import "testing"
+
+var testcases = []struct {
+	root    *BST
+	isValid bool
+}{
+	{&BST{}, true},
+	{&BST{10, &BST{1, nil, nil}, &BST{20, nil, nil}}, true},
+	{&BST{10, &BST{20, nil, nil}, &BST{1, nil, nil}}, false},
+}
+
+func TestIsValidBST(t *testing.T) {
+	t.Parallel()
+	for _, tc := range testcases {
+		if result := IsValidBST(tc.root); result != tc.isValid {
+			t.Errorf("Expected %v got %v", tc.isValid, result)
+		}
+	}
+}
+
+func BenchmarkIsValidBST(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, tc := range testcases {
+			IsValidBST(tc.root)
+		}
+	}
+}