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对于类分配，我需要向提供的 BinarySearchTree 类添加一个方法，该方法将通过将值按顺序存储在数组中来平衡二叉搜索树，并使用这些值构造一棵新树。但是，当我尝试运行该方法时，我得到一个 nullPointerException。如何更改方法以正确平衡二叉搜索树？

我在下面包含了我的代码（试图将其缩减为仅解决问题所需的代码）；底部的两种方法是我试图用于平衡的方法。

package ch08;

import ch05.queues.*;
import java.util.ArrayList;

public class BinarySearchTree<T extends Comparable<T>> implements BSTInterface<T>{

protected BSTNode<T> root;      // reference to the root of this BST
protected LinkedUnbndQueue<T> inOrderQueue; 
protected ArrayList<T> balanceArray;

public BinarySearchTree(){
    root = null;
}

public int reset(int orderType){
    int numNodes = size();

    if (orderType == INORDER){
        inOrderQueue = new LinkedUnbndQueue<T>();
        inOrder(root);
    }
    return numNodes;
}

public T getNext (int orderType){
    if (orderType == INORDER)
        return inOrderQueue.dequeue();
}

    public void balanceTree() {
    int count = reset(INORDER);

    for(int i = 0; i < count; i++) {
        balanceArray.add(getNext(INORDER));
    }
    BinarySearchTree<T> tree = new BinarySearchTree<T>();
    tree.insertTree(0, count - 1);
    this.root = tree.root;
}

public void insertTree(int low, int high){
    if(low == high) {
        add(balanceArray.get(low));
    }
    else if((low + 1) == high) {
        add(balanceArray.get(low));
        add(balanceArray.get(high));
    }
    else {
        int mid = (low + high) / 2;
        add(balanceArray.get(mid));
        insertTree(low, mid - 1);
        insertTree(mid + 1, high);
    }
 }
}
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1 回答 1

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我猜 NullPointerException 来自于您从未初始化balanceArray.

您不应将inOrderQueueand声明balanceArray为字段（恕我直言），因为它们仅与一项操作有关。我会在那里使用论点。

我没有看到类BSTNode，所以我认为它是这样的：

public class BSTNode<T extends Comparable<T>> {
    T value;
    BSTNode<T> left;
    BSTNode<T> right;

    public BSTNode(T value, BSTNode<T> left, BSTNode<T> right) {
        this.value = value;
        this.left = left;
        this.right = right;
    }
}

以下是我将如何进行平衡：

public class BinarySearchTree<T extends Comparable<T>> {
    protected BSTNode<T> root;      // reference to the root of this BST

    public BinarySearchTree() {
        root = null;
    }

    // creates a tree from a sorted list
    private BinarySearchTree(List<T> list) {
        root = buildTree(list, 0, list.size());
    }

    public BinarySearchTree<T> balancedTree()  {
        List<T> list = new ArrayList<>();
        inOrder(root, list);
        return new BinarySearchTree(list);
    }

    private void inOrder(BSTNode<T> node, List<T> list) {
        if (node != null) {
            inOrder(node.left, list);
            list.add(node.value);
            inOrder(node.right, list);            
        }
    }

    private BSTNode<T> buildTree(List<T> list, int i, int size) {
        if (size == 0) {
            return null;
        } else {
            int half = size/2;
            int mid = i+half;
            return new BSTNode<T>(list.get(mid),
                    buildTree(list, i, half),
                    buildTree(list, mid+1, size-half-1));
        }
    }

    ...
}
于 2017-12-14T12:36:12.003 回答