Update and rename generateTrees.cpp to unique-binary-search-trees-ii.cpp

C++/unique-binary-search-trees-ii.cpp

@@ -0,0 +1,57 @@
+// Time:  O(n * 4^n / n^(3/2)) ~= Catalan numbers
+// Space: O(n)
+
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
+ * };
+ */
+class Solution {
+public:
+    vector<TreeNode*> generateTrees(int n) {
+        if (n == 0) {
+            return {};
+        }
+        return generateTreesHelper(1, n);
+    }
+
+private:
+    vector<TreeNode *> generateTreesHelper(int start, int end) {
+        vector<TreeNode *> result;
+        if (start > end) {
+            result.emplace_back(nullptr);
+            return result;
+        }
+
+        for (int i = start; i <= end; ++i) {
+            vector<TreeNode *> leftSubTrees = generateTreesHelper(start, i - 1);
+            vector<TreeNode *> rightSubTrees = generateTreesHelper(i + 1, end);
+            for (const auto& left : leftSubTrees) {
+                for (const auto& right : rightSubTrees) {
+                    TreeNode *root = new TreeNode(i);
+                    root->left = clone(left);
+                    root->right = clone(right);
+                    result.emplace_back(root);
+                }
+            }
+
+        }
+        return result;
+    }
+
+    TreeNode *clone(TreeNode *root) {
+        TreeNode *newRoot = nullptr;
+
+        if (root) {
+            newRoot = new TreeNode(root->val);
+            newRoot->left = clone(root->left);
+            newRoot->right = clone(root->right);
+        }
+
+        return newRoot;
+    }
+};