diff --git a/BinaryTreePreOrderInOrderTraversal.java b/BinaryTreePreOrderInOrderTraversal.java
new file mode 100644
index 00000000..91878aa7
--- /dev/null
+++ b/BinaryTreePreOrderInOrderTraversal.java
@@ -0,0 +1,31 @@
+
+//Time Complexity : O(n2)
+//Space Complexity: O(1)
+class BinaryTreePreOrderInOrderTraversal {
+    public TreeNode buildTree(int[] preorder, int[] inorder)
+    {
+        //PreOrder - root, left, right
+        //InOrder  - left, root, right
+        if(preorder.length == 0) return null;
+        int idx = -1;
+        int rootVal = preorder[0]; // first element in preOrder is nothing but root
+        TreeNode root = new TreeNode(rootVal);
+
+        //Finding the index of the root value in the inorder array
+        for(int i = 0; i < inorder.length; i++){
+            if(inorder[i] == rootVal){
+                idx = i;
+                break;
+            }
+        }
+
+        int[] preLeft = Arrays.copyOfRange(preorder, 1, idx+1);
+        int[] preRight = Arrays.copyOfRange(preorder, idx+1, preorder.length);
+        int[] inLeft = Arrays.copyOfRange(inorder, 0, idx);
+        int[] inRight = Arrays.copyOfRange(inorder, idx+1, inorder.length);
+
+        root.left  = buildTree(preLeft, inLeft);
+        root.right = buildTree(preRight, inRight);
+        return root;
+    }
+}
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diff --git a/ValidBST.java b/ValidBST.java
new file mode 100644
index 00000000..b95a3042
--- /dev/null
+++ b/ValidBST.java
@@ -0,0 +1,46 @@
+/**
+ * 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;
+ *     }
+ * }
+ */
+
+//Time Complexity : O(n)
+//Space Complexity: O(h) for skewed where 'h' is the height of the tree
+//Space Complexity: O(log n) for non-skewed
+class ValidBST {
+
+    // Initialize global variables
+    boolean isValidBST = true;
+    TreeNode prev = null;
+
+    public boolean isValidBST(TreeNode root) {
+        helper(root);
+        return isValidBST;
+    }
+
+    private void helper(TreeNode root)
+    {
+        //Base case
+        if(root == null) return;
+
+        helper(root.left);
+
+        // Check if current root value is less than or equal to previous root value
+        if (prev != null && prev.val >= root.val){
+            isValidBST = false;
+        }
+        prev = root;
+
+        helper(root.right);
+    }
+}
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