public static void main(String[] args) {
TreeNode n1 = new TreeNode(3);
TreeNode n2 = new TreeNode(9);
TreeNode n3 = new TreeNode(20);
n1.left = n2;
n1.right = n3;
zigzagLevelOrder2(n1);
}
public static void main(String[] args) {
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);
Validate_binary_search_tree vbs = new Validate_binary_search_tree();
System.out.println(vbs.isValidBST(root)); // false
}