added unique binary search trees

Author: mihirs16

14. Questions/leetcode 96 - unique binary search trees.py

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+# unique binary search trees | leetcode 96 | https://leetcode.com/problems/unique-binary-search-trees/
+# method: dp, use cached results for subtrees of all possible roots
+
+class Solution:
+    def numTrees(self, n: int) -> int:
+        # cache of possible trees
+        possibleTrees = [1] * (n + 1)
+        
+        # for each number of nodes
+        for numNodes in range(2, n + 1):
+            
+            # for each possible root
+            possibleSubTrees = 0
+            for possibleRoot in range(1, numNodes + 1):
+                Left = possibleRoot - 1
+                Right = numNodes - possibleRoot
+                possibleSubTrees += possibleTrees[Left] * possibleTrees[Right]
+            possibleTrees[numNodes] = possibleSubTrees
+            
+        return possibleTrees[n]
+