我正在进行一项任务，其中我应该创建一个二叉树并从其抽象超类（AbstractBinaryTree.java）定义给定的函数。
在处理一个名为getNumbers()的函数时，它基本上将遍历整个树，同时将每个节点的值添加到它返回的数组列表中。在我的一个if语句中似乎有一个空指针。
AbstractBinaryTree.java
import java.util.ArrayList;
public abstract class AbstractBinaryTree
{
protected Node root;
protected int sizeOfTree;
public AbstractBinaryTree()
{
root = null;
sizeOfTree = 0;
}
public int size(){ return sizeOfTree; }
/** compute the depth of a node */
public abstract int depth(Node node);
/** Check if a number is in the tree or not */
public abstract boolean find(Integer i);
/** Create a list of all the numbers in the tree. */
/* If a number appears N times in the tree then this */
/* number should appear N times in the returned list */
public abstract ArrayList<Integer> getNumbers();
/** Adds a leaf to the tree with number specifed by input. */
public abstract void addLeaf(Integer i);
/** Removes "some" leaf from the tree. */
/* If the tree is empty should return null */
public abstract Node removeLeaf();
// these methods are only needed if you wish
// use the TreeGUI visualization program
public int getheight(Node n){
if( n == null) return 0;
return 1 + Math.max(
getheight(n.getLeft()) , getheight(n.getRight())
);
}
public int height(){ return getheight(root); }
}
Node.java文件。
public class Node{
protected Integer data;
protected Node left;
protected Node right;
public Node(Integer data)
{
this.data = data;
this.left = this.right = null;
}
public Node(Integer data, Node left, Node right){
this.data = data;
this.left = left;
this.right = right;
}
public Integer getData(){ return this.data; }
public Node getLeft(){ return this.left; }
public Node getRight(){ return this.right; }
public void setLeft(Node left){ this.left = left; }
public void setRight(Node right){ this.right = right; }
public void setData(Integer data){ this.data = data; }
}
BinaryTree.java
import java.util.ArrayList;
import java.util.*;
// Student Name: Adrian Robertson
// Student Number: 101020295
//
// References: Collier, R. "Lectures Notes for COMP1406C- Introduction to Computer Science II" [PDF documents]. Retrieved from cuLearn: https://www.carleton.ca/culearn/(Winter2016).//
// References: http://codereview.stackexchange.com/questions/13255/deleting-a-node-from-a-binary-search-tree
// http://www.algolist.net/Data_structures/Binary_search_tree/Removal
// http://www.geeksforgeeks.org/inorder-tree-traversal- without-recursion-and-without-stack/
public class BinaryTree extends AbstractBinaryTree
{
protected Node root = new Node(12);
public static BinaryTree create()
{
BinaryTree tempTree = new BinaryTree();
//creating all the nodes
Node temp10 = new Node(10);
Node temp40 = new Node(40);
Node temp30 = new Node(30);
Node temp29 = new Node(29);
Node temp51 = new Node(51);
Node temp61 = new Node(61);
Node temp72 = new Node(72);
Node temp31 = new Node(31);
Node temp32 = new Node(32);
Node temp42 = new Node(42);
Node temp34 = new Node(34);
Node temp2 = new Node(2);
Node temp61x2 = new Node(61);
Node temp66 = new Node(66);
Node temp3 = new Node(3);
Node temp73 = new Node(73);
Node temp74 = new Node(74);
Node temp5 = new Node(5);
//setting up the tree
if (tempTree.root.getData() == null)
{
tempTree.root.setData(12);
tempTree.root.setLeft(temp10);
tempTree.root.setRight(temp40);
}
temp10.setLeft(temp30);
temp30.setRight(temp29);
temp29.setRight(temp51);
temp51.setLeft(temp61);
temp51.setRight(temp72);
temp40.setLeft(temp31);
temp31.setLeft(temp42);
temp31.setRight(temp34);
temp34.setLeft(temp61x2);
temp61x2.setLeft(temp66);
temp61x2.setRight(temp73);
temp40.setRight(temp32);
temp32.setRight(temp2);
temp2.setLeft(temp3);
temp3.setRight(temp74);
temp74.setLeft(temp5);
return tempTree;
}
public int depth(Node node)
{
Node current = this.root;
int counter = 1;
while(node != current)
{
if (node.getData() > current.getData())
current = current.getRight();
if (node.getData() < current.getData())
current = current.getLeft();
}
return counter;
}
public boolean find(Integer i)
{
boolean found = false;
Node current = this.root;
if (i == current.getData())
found = true;
while (i != current.getData())
{
if (i > current.getData())
current = current.getRight();
if (i < current.getData())
current = current.getLeft();
if (i == current.getData())
found = true;
}
return found;
}
public ArrayList<Integer> getNumbers()
{
ArrayList<Integer> temp = new ArrayList<Integer>();
Node current = this.root;
Node Pre = new Node(null);
while (current.getData() != null )
{
if (current.getLeft().getData() == null)
{
temp.add(current.getData());
current = current.getRight();
}
else
{
/* Find the inorder predecessor of current */
Pre = current.getLeft();
while(Pre.getRight() != null && Pre.getRight() != current)
Pre = Pre.getRight();
/* Make current as right child of its inorder predecessor */
if (Pre.getRight() == null)
{
Pre.setRight(current);
current = current.getLeft();
}
/* Revert the changes made in if part to restore the original tree i.e., fix the right child of predecssor */
else
{
Pre.setRight(null);
temp.add(current.getData());
current = current.getRight();
}/* End of if condition Pre.right == NULL */
}/* End of if condition current.left == NULL*/
}/*End of while */
Collections.sort(temp);
return temp;
}
public void addLeaf(Integer i)
{
insert(this.root, i);
}
public static void insert(Node node, int value) //insert a node Based on provided argument where node is the root of tree
{
if (node == null)
{
Node first = new Node(value);
node = first;
}
else if (value < node.getData())
{
if (node.left != null)
{
insert(node.left, value);
}
else
{
System.out.println(" > Inserted " + value + " to left of node " + node.getData());
Node newNode = new Node(value);
node.left = newNode;
}
}
else if (value > node.getData())
{
if (node.right != null)
{
insert(node.right, value);
}
else
{
System.out.println(" > Inserted " + value + " to right of node " + node.getData());
Node newNode = new Node(value);
node.right = newNode;
}
}
}
public Node removeLeaf()
{
Node tempA = new Node(61); //create a new node with that value
deleteNodeBST(this.root, 61); //delete the node containing that leaf value
return tempA; //return the copy of that node
}
//delete given node with given value
public boolean deleteNodeBST(Node node, int data) {
ArrayList<Integer> temp = this.getNumbers();
if (node == null) {
return false;
}
if (node.getData() == data) {
if ((node.getLeft() == null) && (node.getRight() == null)) {
// leaf node
node = null;
return true;
}
if ((node.getLeft() != null) && (node.getRight() != null)) {
// node with two children
node.setData(temp.get(0));
return true;
}
// either left child or right child
if (node.getLeft() != null) {
this.root.setLeft(node.getLeft());
node = null;
return true;
}
if (node.getRight() != null) {
this.root.setRight(node.getRight());
node = null;
return true;
}
}
this.root = node;
if (node.getData() > data) {
return deleteNodeBST(node.getLeft(), data);
} else {
return deleteNodeBST(node.getRight(), data);
}
}
public static void main(String args[])
{
BinaryTree myTree = new BinaryTree();
myTree.create();
System.out.println(myTree.getNumbers());
}
}
create函数创建一个二叉树并返回该二叉树。这是我应该根据分配指南创建的预定义二叉树。我知道树值没有正确组织，因为它们在适当的二叉树中。这是否在遍历期间导致空指针？导致遍历被taylored用于正确的二叉树。
答案 0 :(得分：0)
请相应地更新getNumbers - method，
您需要在使用引用类型之前进行正确的检查。
public ArrayList<Integer> getNumbers()
{
ArrayList<Integer> temp = new ArrayList<Integer>();
Node current = this.root;
Node Pre = new Node(null);
while (current != null && current.getData() != null ) // Fix here... Add : current != null
{
if (current.getLeft() != null && current.getLeft().getData() == null) // Fix here... Add : current.getLeft() != null
{
temp.add(current.getData());
current = current.getRight();
}
else
{
/* Find the inorder predecessor of current */
Pre = current.getLeft();
while(Pre != null && Pre.getRight() != null && Pre.getRight() != current) // Fix here... Add : Pre != null
Pre = Pre.getRight();
/* Make current as right child of its inorder predecessor */
if (Pre != null && Pre.getRight() == null) // Fix here... Add : Pre != null
{
Pre.setRight(current);
current = current.getLeft();
}
/* Revert the changes made in if part to restore the original tree i.e., fix the right child of predecssor */
else
{
if(Pre != null){ // Fix here... Add : Pre != null
Pre.setRight(null);
}
temp.add(current.getData());
current = current.getRight();
}/* End of if condition Pre.right == NULL */
}/* End of if condition current.left == NULL*/
}/*End of while */
Collections.sort(temp);
return temp;
}
答案 1 :(得分：0)
在BinaryTree类中，只有在没有数据时才初始化根节点的左侧和右侧。但是根节点是用数据创建的......
您应该在以下情况中反转条件：
//setting up the tree
if (tempTree.root.getData() == null)
在getNumbers（）中添加测试：
if (current.getLeft() == null || current.getLeft().getData() == null)
答案 2 :(得分：0)
在BinaryTree类，getNumbers()方法和while循环中。也许你的问题在这里：
if (current.getLeft().getData() == null) {
temp.add(current.getData());
current = current.getRight();
}
当您致电current.getLeft()时，当null节点为left时，它会返回null。然后，您致电getData()它会抛出NullPointerException。如果您不确定它始终不为null，请在调用它的任何方法之前检查它。您可以将if语句更改为：
if (current.getLeft() != null && current.getLeft().getData() == null) {
temp.add(current.getData());
current = current.getRight();
}
或者：
Node left = current.getLeft();
if (left == null) {
//TODO something here
} else if (left.getData() == null) {
temp.add(current.getData());
current = current.getRight();
}