Java2叉排序树

[云淡风起]

public class TreeTest {
	private Node root = null;	
	public static void main(String[] args) {	
		TreeTest hw = new TreeTest();	
		for (int i = 0; i < 5; i++) 
			hw.add(i);
		hw.preOrder();
	}	
	public void add(int i) {
		insert(root,i);
	}
	public void insert(Node root,int data) {
		if (root == null){
			root = new Node(data);
		}
		else if (root.data > data)
			 insert(root.left, data);
		else
			insert(root.right, data);		
	}
	public void preOrder() {
		preOrder(root);
	}
	public void preOrder(Node root) {
	
		if (root == null){
			System.out.println("null");
			return;
		}
		System.out.println(root.data);
		preOrder(root.left);
		preOrder(root.right);
	}
///
	 class Node {
			int data;
			Node left = null;
			Node right = null;

			public Node() {
			}

			public Node(int data) {
				this.data = data;
			}
		}

}

[云淡风起]

算法有问题,求解释.