Create ValidateBST.java

Author: techieJarek

ValidateBST.java

@@ -0,0 +1,51 @@
+/**
+ * Given a binary tree, determine if it is a valid Binary Search Tree (BST).
+ */
+public class ValidateBST {
+    private static TreeNode previousVisitedNode;
+    public static void main(String[] args){
+        System.out.println("Tree is valid = " + isValidBST(getValidTree()));
+        System.out.println("Tree is valid = " + isValidBST(getInValidTree()));
+    }
+
+    public static class TreeNode {
+        int val;
+        TreeNode left;
+        TreeNode right;
+        TreeNode(int x) { val = x; }
+    }
+
+    public static TreeNode getValidTree(){
+        TreeNode root = new TreeNode(10);
+        root.left = new TreeNode(5);
+        root.right = new TreeNode(15);
+        root.right.left = new TreeNode(13);
+        root.right.right = new TreeNode(20);
+        return root;
+    }
+
+    public static TreeNode getInValidTree(){
+        TreeNode root = new TreeNode(10);
+        root.left = new TreeNode(5);
+        root.right = new TreeNode(15);
+        root.right.left = new TreeNode(6);
+        root.right.right = new TreeNode(20);
+        return root;
+    }
+
+    public static boolean isValidBST(TreeNode root) {
+        return isInIncreasingOrder(root);
+    }
+
+    private static boolean isInIncreasingOrder(TreeNode root) {
+        if(root == null) return true;
+
+        if(isInIncreasingOrder(root.left)){
+            if(previousVisitedNode!=null && previousVisitedNode.val > root.val) return false;
+            previousVisitedNode = root;
+            return isInIncreasingOrder(root.right);
+        } else {
+            return false;
+        }
+    }
+}