public void visit(TreeNode node){
    	if(node.left != null) visit(node.left);
    	if(first){
    		first = false;
    	} else {
    		if(preNode.val > node.val){
    			if(node1 == null) node1 = preNode;
    			node2 = node;
    		}
    	}
    	preNode = node;
    	if(node.right != null) visit(node.right);
    }