我正在尝试编写二叉树。除非我尝试删除根节点，否则一切正常。
返回以下错误。我不知道如何解决这个问题。请帮忙。
Add，Statistics，Balance等所有其他功能都可以正常使用。只有当我尝试在根节点上执行删除时，程序才会返回。
Exception in thread "main" java.lang.NullPointerException
at TreeClass$BinarySearchTree.Delete(TreeClass.java:147)
at TreeClass$BinarySearchTree.fnHandler(TreeClass.java:292)
at TreeClass.main(TreeClass.java:341)
但是，删除适用于树中的其他节点。只会遇到根节点的问题。 这是我的代码
import java.util.*;
public class TreeClass
{
//--------------------------------------------------------------
static Scanner scanner = new Scanner(System.in);
static BinarySearchTree BST = new BinarySearchTree();
//--------------------------------------------------------------
public static class TreeNode
{
TreeNode Parent;
TreeNode LChild;
TreeNode RChild;
Integer Data;
public TreeNode(int cbData)
{
this.Data = cbData;
this.LChild = null;
this.RChild = null;
this.Parent = null;
}
public void SetParent(TreeNode parent)
{
this.Parent = parent;
}
}
public static class BinarySearchTree
{
int size;
TreeNode Root;
public BinarySearchTree()
{
this.size = 0;
this.Root = null;
}
public void Allocate(TreeNode node, ArrayList<Integer> DataList)
{
if (node.LChild != null)
Allocate(node.LChild, DataList);
DataList.add(node.Data);
if (node.RChild != null)
Allocate(node.RChild, DataList);
}
public void Add(int cbData)
{
TreeNode temp = null;
TreeNode node = new TreeNode(cbData);
if(this.size == 0)
{
this.Root = node;
}
else
{
temp = this.Root;
while(temp != null)
{
if(cbData > temp.Data)
{
if(temp.RChild == null)
{
temp.RChild = node;
node.Parent = temp;
break;
}
temp = temp.RChild;
}
else if(cbData < temp.Data)
{
if(temp.LChild == null)
{
temp.LChild = node;
node.Parent = temp;
break;
}
temp = temp.LChild;
}
else
{
//System.out.printf("[Error] %d already exists!\n", cbData);
return;
}
}
}
if(size == 0)
System.out.printf("[Success] %d has been added as root\n", node.Data);
else
System.out.printf("[Success] %d has been added - Parent: %d\n", node.Data, node.Parent.Data);
++this.size;
}
public void AddReBalance(int[] List, int min, int max)
{
if(min <= max)
{
int current = (max+min)/2;
Add(List[current]);
AddReBalance(List, min, current-1);
AddReBalance(List, current+1, max);
}
}
public void Delete(int cbData)
{
TreeNode temp = null;
if(this.size > 0)
{
temp = this.Root;
while(temp != null)
{
if(cbData > temp.Data)
temp = temp.RChild;
else if(cbData < temp.Data)
temp = temp.LChild;
else
{
System.out.printf("[Success] %d found and deleted!\n", cbData);
if(temp.LChild != null)
{
temp.LChild.Parent = temp.Parent;
if(temp == temp.Parent.RChild)
temp.Parent.RChild = temp.LChild;
else if(temp == temp.Parent.LChild)
temp.Parent.LChild = temp.LChild;
}
else if(temp.RChild != null)
{
temp.RChild.Parent = temp.Parent;
if(temp == temp.Parent.LChild)
temp.Parent.LChild = temp.RChild;
else if(temp == temp.Parent.RChild)
temp.Parent.RChild = temp.RChild;
}
else
{
if(temp == temp.Parent.LChild)
temp.Parent.LChild = null;
else if(temp == temp.Parent.RChild)
temp.Parent.RChild = null;
temp.Parent = null;
}
--this.size;
return;
}
}
}
System.out.printf("[Error] %d not found!\n", cbData);
}
public int Find(int cbData)
{
int Level = 0;
TreeNode temp = this.Root;
while(temp != null)
{
if(cbData > temp.Data)
{
temp = temp.RChild;
++Level;
}
else if(cbData < temp.Data)
{
temp = temp.LChild;
++Level;
}
else if(cbData == temp.Data)
{
return ++Level;
}
}
return -1;
}
public void Rebalance()
{
int[] cbList = new int[this.size];
ArrayList<Integer> DataList = new ArrayList();
Allocate(this.Root, DataList);
for(int i = 0; i < DataList.size(); ++i)
cbList[i] = DataList.get(i);
this.size = 0;
if(cbList.length > 0)
AddReBalance(cbList, 0, cbList.length-1);
else
System.out.print("[Error] You do not have any nodes to balance!\n");
}
public void DisplayContent(TreeNode node)
{
ArrayList<Integer> DataList = new ArrayList();
Allocate(this.Root, DataList);
System.out.printf("Tree Content: ");
for(int i = 0; i < DataList.size(); ++i)
System.out.printf("%d ", DataList.get(i));
System.out.printf("\n");
}
public void GetPathData(TreeNode node, ArrayList<Integer> DepthList, int Level)
{
++Level;
if(node.RChild != null | node.LChild != null)
{
if (node.LChild != null)
GetPathData(node.LChild, DepthList, Level);
if (node.RChild != null)
GetPathData(node.RChild, DepthList, Level);
}
else
{
DepthList.add(Level);
Level = 0;
}
}
public void DisplayStats()
{
int Level = 0, Max = 0, Min = 0;
ArrayList<Integer> DList = new ArrayList();
GetPathData(this.Root, DList, Level);
for(int i = 0; i < DList.size(); ++i)
{
int TempPath = DList.get(i);
if(i == 0)
{
Min = TempPath;
Max = TempPath;
}
else
{
if(Min > TempPath)
Min = TempPath;
if(Max < TempPath)
Max = TempPath;
}
}
System.out.printf("Root Data: %d\nItem Count: %d\nShortest Path: %d" +
"\nLongest Path: %d\n\n", this.Root.Data, this.size, Min, Max);
}
public boolean fnHandler(int cbIn)
{
int cbInItem = 0;
int cbTempRetn = 0;
boolean retn = false;
switch(cbIn)
{
case 1:
System.out.printf("Input an integer to add: ");
cbInItem = scanner.nextInt();
Add(cbInItem);
break;
case 2:
System.out.printf("Input an integer to delete: ");
cbInItem = scanner.nextInt();
Delete(cbInItem);
break;
case 3:
System.out.printf("Input an integer to find:");
cbInItem = scanner.nextInt();
cbTempRetn = BST.Find(cbInItem);
if(cbTempRetn == -1)
System.out.printf("[Error] This item was not found!\n");
else
System.out.printf("[Success] Item was found on level: %d\n", cbTempRetn);
break;
case 4:
BST.Rebalance();
System.out.printf("[Success] Tree has been balanced!\n");
break;
case 5:
BST.DisplayContent(BST.Root);
break;
case 6:
BST.DisplayStats();
break;
case 7:
retn = true;
System.out.printf("[Exitting...]\n");
break;
}
return retn;
}
}
public static void main(String[] args)
{
int cbInput = 0;
boolean exit = false;
while(exit == false)
{
System.out.printf("\n1. Add Entry - 2. Delete Item - 3. Find Item - 4. Balance Tree"
+ "\n5. List Data - 6. Statistics - 7. Exit\n\nInput: ");
cbInput = scanner.nextInt();
exit = BST.fnHandler(cbInput);
}
}
}
答案 0 :(得分：2)
你遇到的问题不仅仅是空指针异常......
删除节点时，您尝试获取剩余的“子节点”并将其重新附加到已删除节点的父节点上：
if(temp.LChild != null)
{
temp.LChild.Parent = temp.Parent;
if(temp == temp.Parent.RChild)
temp.Parent.RChild = temp.LChild;
else if(temp == temp.Parent.LChild)
temp.Parent.LChild = temp.LChild;
}
而且，在你的情况下，temp.Parent是null（因为它是根节点），因此你得到了NPE（像......这样的行不一定是这样的行）：
temp == temp.Parent.RChild
不幸的是，你在这里遇到了一些问题......
当您删除一般情况下的节点（即不是根）时，父节点可能已经在另一条腿上有子节点，因此您无法将已删除节点的子节点都附加到父节点....
...这比解释中容易解释更复杂，但是，底线是你删除的代码不只是删除一个节点，而是删除节点和其中一个节点上的所有子节点腿。
因此，当您解决null Parent时，您会发现还有其他问题。