From f9e9a910b7acce4c4bec3e8b098ae9b239356ea7 Mon Sep 17 00:00:00 2001
From: Gleb Koval <gleb@koval.net>
Date: Mon, 8 Nov 2021 12:56:51 +0000
Subject: [PATCH] unique-binary-search-trees

---
 unique-binary-search-trees/sol.go | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)
 create mode 100644 unique-binary-search-trees/sol.go

diff --git a/unique-binary-search-trees/sol.go b/unique-binary-search-trees/sol.go
new file mode 100644
index 0000000..64d28cc
--- /dev/null
+++ b/unique-binary-search-trees/sol.go
@@ -0,0 +1,26 @@
+package main
+
+// Time: O(n^2)
+// Space: O(n)
+func numTrees(n int) int {
+	if n < 3 {
+		return n
+	}
+	trees := map[int]int{
+		0: 1,
+		1: 1,
+		2: 2,
+	}
+	for c := 3; c <= n; c++ {
+		s := 0
+		for p := 0; p < c/2; p++ {
+			s += trees[p] * trees[c-p-1] * 2
+		}
+		if (c-1)%2 == 0 {
+			p := (c - 1) / 2
+			s += trees[p] * trees[p]
+		}
+		trees[c] = s
+	}
+	return trees[n]
+}