diff --git a/top-100-liked/40/leetcode_test.go b/top-100-liked/40/leetcode_test.go
index 789b6b0..7a517a2 100644
--- a/top-100-liked/40/leetcode_test.go
+++ b/top-100-liked/40/leetcode_test.go
@@ -9,5 +9,19 @@ type TreeNode struct {
 }
 
 func diameterOfBinaryTree(root *TreeNode) int {
+	maxDiameter := 0
 
+	var depth func(node *TreeNode) int
+	depth = func(node *TreeNode) int {
+		if node == nil {
+			return 0
+		}
+		left := depth(node.Left)
+		right := depth(node.Right)
+		maxDiameter = max(maxDiameter, left+right)
+		return max(left, right) + 1
+	}
+
+	depth(root)
+	return maxDiameter
 }