Create 0096-unique-binary-search-trees.kt

Author: a93a

kotlin/0096-unique-binary-search-trees.kt

@@ -0,0 +1,41 @@
+//"pure" dp
+class Solution {
+    fun numTrees(n: Int): Int {
+        val cache = IntArray (n + 1) { 1 }
+
+        for (node in 2..n) {
+            var res = 0
+            for (root in 1..node) {
+                val left = root - 1
+                val right = node - root
+                res += cache[left] * cache[right]
+            }
+            cache[node] = res
+        }
+
+        return cache[n]
+    }
+}
+
+//recursion + memoization
+class Solution {
+    fun numTrees(n: Int): Int {
+        val cache = IntArray (n + 1) { -1 }
+
+        fun count(root: Int): Int {
+            if (root == 0) return 1
+            if (cache[root] != -1) return cache[root]
+
+            var res = 0
+            for (left in 0 until root) {
+                val right = root - left - 1
+                res += count(left) * count(right)
+            }
+
+            cache[root] = res
+            return res
+        }
+
+        return count(n)
+    }
+}