在Java中使用递归无法进行BST插入

Question

I am trying to write a recursive way to insert to my binary search tree using java but it is not working properly gives null pointer exception my code for node.java is

public class Node
{
public int data;
public Node left;
public Node right;

public Node()
{
    this.data = -1;
    this.left = null;
    this.right = null;
}
public Node(int n)
{
    this.data = n;
    this.left = null;
    this.right = null;
}
}

my code in Tree.java is

public class Tree
{
public Node head = new Node();

public void insert(int n , Node m)
{
    if(m == null || m.data == -1)
    {
        m = new Node(n);
    }
    else
    {
        if(m.data > n)
        {
            insert(n,m.left);
        }
        else if(m.data < n)
        {
            insert(n,m.right);
        }
    }
}
public void print()
{
    System.out.println(head.data);
    System.out.println(head.left.data);
    System.out.println(head.right.data);
}
}

and test.java code is

public class Test
{
public static void main(String[] args)
{
    Tree t = new Tree();
    Node m = new Node();
    t.insert(12,t.head);
    t.insert(11,t.head);
    t.insert(13,t.head);
    t.print();
}
}

when I compile and run, it gives following error

-1
Exception in thread "main" java.lang.NullPointerException
at Tree.print(Tree.java:28)
at Test.main(Test.java:10)

Answer 1

A few issues with the code as written:

1. The base case ( if block) of your insert doesn't actually modify the tree. It simply creates a new node and exits without linking the new node to the tree.
2. t.head starts with data = -1 , so any subsequent call to t.insert(..., t.head) will hit the if block in insert() and do nothing, as mentioned in issue 1.
3. The NullPointerException doesn't happen until you try t.print() where you read head.left.data , but since you only have a single node tree at this point, head.left is null.

The core problem is your insert method doesn't add nodes properly. It needs to actually add the new node as a subtree of the current node.

Here's the fixed code (could probably be simplified):

public class Tree {
    public Node head = new Node();

    public void insert(int n , Node m)
    {
        if(m == head && head.data == -1) 
        {
            // if we have an empty tree, just change the head
            head.data = n;
        }
        else if(m.data > n) {
            // we should be adding to the left subtree here
            if(m.left == null) {
                // if no left subtree, create a new node and link to m's left
                m.left = new Node(n);
            }
            else {
                // otherwise call insert on left subtree recursively
                insert(n, m.left);
            }
        }
        else
        {
            // otherwise add to right subtree
            if(m.right == null) {
                // no right subtree, so create node and link to m's right
                m.right = new Node(n);
            }
            else {
                // call recursively
                insert(n, m.right);
            }
        }
    }
    public void print()
    {
        System.out.println(head.data);
        System.out.println(head.left.data);
        System.out.println(head.right.data);
    }
}