add: BinarySearchTreeValidation, use null to replace float('-inf') and float('inf') in python solution

Author: Jer3myYu

java/Trees/BinarySearchTreeValidation.java

@@ -0,0 +1,39 @@
+import DS.TreeNode;
+
+/*
+    // Definition of TreeNode:
+    class TreeNode {
+        public int val;
+        public TreeNode left;
+        public TreeNode right;
+        public TreeNode(int val) {
+            this.val = val;
+        }
+    }
+ */
+
+public class BinarySearchTreeValidation {
+    public boolean binarySearchTreeValidation(TreeNode root) {
+        // Start validation at the root node. The root node can contain any 
+        // value, so set the initial lower and upper bounds to null.
+        return isWithinBounds(root, null, null);
+    }
+
+    private boolean isWithinBounds(TreeNode node, Integer lowerBound, Integer upperBound) {
+        // Base case: if the node is null, it satisfies the BST condition.
+        if (node == null) {
+            return true;
+        }
+        // If the current node's value is not within the valid bounds, this 
+        // tree is not a valid BST.
+        if (lowerBound != null && node.val <= lowerBound || upperBound != null && upperBound <= node.val) {
+            return false;
+        }
+        // If the left subtree isn't a BST, this tree isn't a BST.
+        if (!isWithinBounds(node.left, lowerBound, node.val)) {
+            return false;
+        }
+        // Otherwise, return true if the right subtree is also a BST.
+        return isWithinBounds(node.right, node.val, upperBound);
+    }
+}