day 2

Author: sachuverma

Leetcode Daily Challenge/September-2021/02. Unique Binary Search Trees II.cpp

@@ -0,0 +1,67 @@
+/*
+Unique Binary Search Trees II
+=============================
+
+Given an integer n, return all the structurally unique BST's (binary search trees), which has exactly n nodes of unique values from 1 to n. Return the answer in any order.
+
+Example 1:
+Input: n = 3
+Output: [[1,null,2,null,3],[1,null,3,2],[2,1,3],[3,1,null,null,2],[3,2,null,1]]
+
+Example 2:
+Input: n = 1
+Output: [[1]]
+ 
+Constraints:
+1 <= n <= 8
+*/
+
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
+ * };
+ */
+
+class Solution
+{
+public:
+    vector<TreeNode *> dfs(int start, int end)
+    {
+        if (start > end)
+            return {NULL};
+        if (start == end)
+            return {new TreeNode(start)};
+
+        vector<TreeNode *> ans;
+
+        for (int i = start; i <= end; ++i)
+        {
+            auto left = dfs(start, i - 1);
+            auto right = dfs(i + 1, end);
+
+            for (auto &l : left)
+            {
+                for (auto &r : right)
+                {
+                    TreeNode *root = new TreeNode(i);
+                    root->left = l;
+                    root->right = r;
+                    ans.push_back(root);
+                }
+            }
+        }
+
+        return ans;
+    }
+
+    vector<TreeNode *> generateTrees(int n)
+    {
+        return dfs(1, n);
+    }
+};

Leetcode Daily Challenge/September-2021/README.md

@@ -1,6 +1,7 @@
 # September 2021 LeetCoding Challenge
 
-| Day | Question Links                                                                                                                                                   |                              Solutions                               |
-| :-: | :--------------------------------------------------------------------------------------------------------------------------------------------------------------- | :------------------------------------------------------------------: |
-|     | []()  |   [cpp](./.cpp)    |
-| 1.  | [Array Nesting](https://leetcode.com/explore/featured/card/september-leetcoding-challenge-2021/636/week-1-september-1st-september-7th/3960/) | [cpp](./01.%20Array%20Nesting.cpp) |
+| Day | Question Links                                                                                                                                                |                        Solutions                         |
+| :-: | :------------------------------------------------------------------------------------------------------------------------------------------------------------ | :------------------------------------------------------: |
+|     | []()                                                                                                                                                          |                      [cpp](./.cpp)                       |
+| 1.  | [Array Nesting](https://leetcode.com/explore/featured/card/september-leetcoding-challenge-2021/636/week-1-september-1st-september-7th/3960/)                  |            [cpp](./01.%20Array%20Nesting.cpp)            |
+| 2.  | [Unique Binary Search Trees II](https://leetcode.com/explore/challenge/card/september-leetcoding-challenge-2021/636/week-1-september-1st-september-7th/3961/) | [cpp](./02.%20Unique%20Binary%20Search%20Trees%20II.cpp) |