From 9579614dd71260c664000dd8b3befdc7e87e8718 Mon Sep 17 00:00:00 2001
From: Sanjoli <sanjolisingh90@gmail.com>
Date: Mon, 29 Jun 2026 20:36:02 -0500
Subject: [PATCH] Completed Trees 1

---
 ConstructBTFromPreAndInOrderTraversal.java | 55 ++++++++++++++++++++++
 ValidBST.java                              | 49 +++++++++++++++++++
 2 files changed, 104 insertions(+)
 create mode 100644 ConstructBTFromPreAndInOrderTraversal.java
 create mode 100644 ValidBST.java

diff --git a/ConstructBTFromPreAndInOrderTraversal.java b/ConstructBTFromPreAndInOrderTraversal.java
new file mode 100644
index 00000000..4b34b541
--- /dev/null
+++ b/ConstructBTFromPreAndInOrderTraversal.java
@@ -0,0 +1,55 @@
+import java.util.HashMap;
+
+import javax.swing.tree.TreeNode;
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode() {}
+ *     TreeNode(int val) { this.val = val; }
+ *     TreeNode(int val, TreeNode left, TreeNode right) {
+ *         this.val = val;
+ *         this.left = left;
+ *         this.right = right;
+ *     }
+ * }
+ */
+class Solution {
+    /*
+        TC - O(n)
+        SC - O(n)
+        Solution- In preorder traversal, the first element is always the root of the current subtree. We are using
+        a HashMap stores the index of each value in the inorder array, allowing us to quickly find the root's position.
+        Starting with the first element in the preorder array, we create the root node and move to the next element 
+         using the global index idx. Using the root's position in the inorder array, we split the tree into the left subtree 
+         (elements before the root in inorder), and the right subtree (elements after the root in inorder).
+    */
+    HashMap<Integer, Integer> map;
+    int idx;
+    public TreeNode buildTree(int[] preorder, int[] inorder) {
+        this.map = new HashMap<>();
+        for (int i = 0; i< inorder.length; i++) {
+            map.put(inorder[i], i);
+        }
+        this.idx = 0;
+        return helper(preorder, 0, preorder.length - 1);
+    }
+
+    private TreeNode helper(int[] preorder, int start, int end) {
+        if (start > end) {
+            return null;
+        }
+        
+        int rootVal = preorder[idx];
+        this.idx++;
+        int rootIdx = map.get(rootVal);
+        
+        TreeNode node = new TreeNode(rootVal);
+        node.left = helper(preorder, start, rootIdx - 1);
+        node.right = helper(preorder, rootIdx + 1, end);
+
+        return node;
+    }
+}
\ No newline at end of file
diff --git a/ValidBST.java b/ValidBST.java
new file mode 100644
index 00000000..279cbb1f
--- /dev/null
+++ b/ValidBST.java
@@ -0,0 +1,49 @@
+import javax.swing.tree.TreeNode;
+
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode() {}
+ *     TreeNode(int val) { this.val = val; }
+ *     TreeNode(int val, TreeNode left, TreeNode right) {
+ *         this.val = val;
+ *         this.left = left;
+ *         this.right = right;
+ *     }
+ * }
+ */
+class Solution {
+    //TC - O(n)
+    //SC - O(h)
+    /*
+        Inthis solition we are using inorder traversal to validate the BST. The previously visited node
+        is saved in the prev variable and the current node's value is compared to the previously visited node.
+        If current value is less or equal to the prev node's value then it is not a valid BST and set the value of flag to
+        false. We use flag to keep track of this and when flag is false, we break out of the tree traversal.
+    */
+    boolean flag;
+    TreeNode prev;
+    public boolean isValidBST(TreeNode root) {
+        flag = true;
+        isValid(root);
+        return flag;
+    }
+
+    private void isValid(TreeNode root) {
+        if (root == null) return;
+        if (!flag) return;
+
+        isValid(root.left);
+
+        if (prev != null && root.val <= prev.val) {
+            flag = false;
+        }
+
+        prev = root;
+
+        isValid(root.right);
+    }
+}
\ No newline at end of file