Added ValidateBinarySearchTree

Java/validateBinarySearchTree.java

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+// Given a binary tree, determine if it is a valid binary search tree (BST).
+
+// Assume a BST is defined as follows:
+
+// The left subtree of a node contains only nodes with keys less than the node's key.
+// The right subtree of a node contains only nodes with keys greater than the node's key.
+// Both the left and right subtrees must also be binary search trees.
+// Example 1:
+//     2
+//    / \
+//   1   3
+// Binary tree [2,1,3], return true.
+// Example 2:
+//     1
+//    / \
+//   2   3
+// Binary tree [1,2,3], return false.
+
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode(int x) { val = x; }
+ * }
+ */
+public class Solution {
+
+    public boolean isValidBST(TreeNode root) {
+
+        if(root == null) return true;
+
+        return validBSTRecursive(root, Long.MIN_VALUE, Long.MAX_VALUE);
+
+    }
+
+    public boolean validBSTRecursive(TreeNode root, long minValue, long maxValue) {
+
+        if(root == null) return true;
+        else if(root.val >= maxValue || root.val <= minValue) return false;
+        else return validBSTRecursive(root.left, minValue, root.val) && validBSTRecursive(root.right, root.val, maxValue);
+
+    }
+
+}