为什么只创建二叉树的右节点？

Question

I'm supposed to create a binary search tree that takes in an integer array. I want to know how can I access the left and right nodes in the Node class to create the left and right nodes.

However, this code only creates the right nodes of the binary tree. What is wrong with my code?

This is the Node class:

private int element;
private BinaryNode left;
private BinaryNode right;

public Node( ){
    this( 0, null, null );
}

public( int theElement, BinaryNode lt, BinaryNode rt ){
    element = theElement;
    left    = lt;
    right   = rt;
}

public int getElement( ){
    return element;
}

public BinaryNode getLeft( ){
    return left;
}

public BinaryNode getRight( ){
    return right;
}

public void setElement( int x ){
    element = x;
}

public void setLeft( BinaryNode t ){
    left = t;
}

public void setRight( BinaryNode t ){
    right = t;
}

And this is my code to create the tree. These two blocks of code are in different java class files:

static Node obj = new Node();
static Node root = new Node();

static Node BinaryTree(int[] array, int begin, int end){
    if(begin > end){  //checks in the array is still in bound
        return null;  //returns null so the next node can be null
    }

    int middle = (begin + end)/2;  //finds the middle index
    obj.setElement(array[middle]);
    root = new Node(obj.getElement(), obj.getLeft(), obj.getRight()); //creates the root node of the tree/subtree
    root.setLeft(BinaryTree(array, begin, middle-1)); //creates the left tree or node recursively
    root.setRight(BinaryTree(array, middle+1, end));  //creates the right tree or node recursively
    return root; //places/returns the node of the tree in its place
}

Answer 1

Some of the issues are:

• Your code has a mix of BinaryNode and Node . I will assume Node .
• The first code block has a constructor without a name -- that is a syntax error.
• root should not be initialised as new Node , but as the return value from a call to BinaryTree
• obj should not be defined in a scope that is larger than the BinaryTree function, as you will keep reusing the same object over and over in each recursive call, which will have devastating effects on the algorithm. You don't even need this variable.

Change BinaryTree to this:

static Node BinaryTree(int[] array, int begin, int end){
    if (begin > end) {
        return null;
    }

    int middle = (begin + end) / 2;
    return new Node(array[middle], BinaryTree(array, begin, middle - 1), 
                                   BinaryTree(array, middle + 1, end));
}

An example call could be like this:

int[] array = {1,2,3,4,5};
Node root = BinaryTree(array, 0, array.length - 1);