private void traverse(TreeNode root)
    {
    	if (root == null) return;
    	traverse(root.left);
    	if (pre == null)
    		pre = root;
    	else
    	{
    		if (pre.val > root.val)
    		{
    			if (first == null)
    				first = pre;
    			second = root;
    		}
    		pre = root;
    	}
    	traverse(root.right);
    }