Create Validate Binary Search Tree.java

Author: sudoshivesh

medium/Validate Binary Search Tree.java

@@ -0,0 +1,18 @@
+//98. Validate Binary Search Tree
+class Solution {
+  public boolean isValidBST(TreeNode root) {
+    return isValidBST(root, null, null);
+  }
+
+  private boolean isValidBST(TreeNode root, TreeNode minNode, TreeNode maxNode) {
+    if (root == null)
+      return true;
+    if (minNode != null && root.val <= minNode.val)
+      return false;
+    if (maxNode != null && root.val >= maxNode.val)
+      return false;
+
+    return isValidBST(root.left, minNode, root) &&
+           isValidBST(root.right, root, maxNode);
+  }
+}