Binary search tree includes 0 in results (Java)

Question

I'm relatively new to Java, and am currently learning about implementing trees in Java, specifically Binary Search trees. Right now, most of what I have done has worked fine (Inorder and Postorder traversals, switching nodes, etc.), except for one important detail: 0 is added to my tree, even though I never inserted it. In other words, when I call for the inorder traversal of 45 9 2 100 4, I get 0 2 4 9 45 100. Here is the relevant code:

From the main function:

Scanner sc=new Scanner(System.in);
int m, n, j;
treenode T1=new treenode();
System.out.println("How many numbers will be entered?");
m=sc.nextInt();
System.out.println("Enter "+m+" integers.");
for(j=0;j<m;j++){
  n=sc.nextInt();
  T1.insert(T1, n);
}
System.out.println("Tree T1, in inorder sequence:");
T1.Inorder(T1);

etc.

And the function calls:

class treenode{
int data; treenode left; treenode right;
public treenode() {};
public treenode(int j)
{data = j; left = null; right = null;}
// Insert node function
public treenode insert(treenode t, int k){
  if (t == null)
  {t = new treenode();
   t.data = k; t.left=null;
   t.right=null;  return t;}
  else{
    if( k > t.data)
  {t.right = insert(t.right,k);
    return t;}
  else{t.left = insert(t.left, k);
    return t;}}}
// Inorder traversal function
public void Inorder(treenode t){
  if(t != null){
    Inorder(t.left);
    System.out.println(" " + t.data);
    Inorder(t.right);}
}

etc.

And yes, I know, I need to change the class name to Treenode so it starts with a capital letter; I'll be doing that shortly.

Answer 1

treenode T1=new treenode();

该行将创建一个没有数据值的根节点（默认为0），因此您始终将0作为树的根。

Answer 2

treenode T1=new treenode();

This creates a node with the value 0, not an empty tree. Representing an empty tree is tricky; the value null won't work with the current implementation because it calls T1.insert() , which will throw a null pointer exception.

One option is to make insert() static by adding the static modifier to the declaration, and changing the call to

T1 = treenode.insert(T1, n);

A similar thing would have to be done for Inorder() , otherwise an explicit null check is required by the caller.

Another option is to create a wrapper class, Tree , which has a root node (that can be null ). This would make the external interface cleaner:

T1.insert(n);
T1.Inorder();

---

As an aside, by convention classes and interfaces start with capital letters in Java while methods and variables start with a lower case letter. Following the convention helps others understand your code.

Answer 3

I went a different route to solve it (but you were both right, the initialization of the tree was the problem). I took the first node out of the for loop to initialize the tree with the first data point as follows:

treenode T1=new treenode(n);
for(j=1;j<m;j++){
  n=sc.nextInt();
  T1.insert(T1, n);
}

And now everything seems to be running smoothly. Thanks!