How to solve: Java data structure error: Linked list loop

How to solve: Java data structure error: linked list loop

Introduction:
In Java programming, linked lists are often used as a data structure to store and Operation data. However, a common mistake sometimes occurs when dealing with linked list operations - linked list loops. A linked list cycle means that a node in the linked list points to the previous node or the next node in the linked list, causing the linked list to not be traversed correctly. This article explores how to solve this problem and provides code examples.

Problem Analysis:
Linked list loops are usually caused by references to linked list nodes pointing to the wrong location. This may be due to coding errors, logic errors, or other reasons. Before solving this problem, let's first look at a code example:

class Node {
    int data;
    Node next;
    
    Node(int data) {
        this.data = data;
        this.next = null;
    }
}

public class LinkedList {
    Node head;
    
    public void add(int data) {
        Node newNode = new Node(data);
        
        if(head == null) {
            head = newNode;
        } else {
            Node current = head;
            
            while(current.next != null) {
                current = current.next;
            }
            
            current.next = newNode;
        }
    }
}

The above code is a simple linked list implementation, which contains a Node class to represent the node, and a LinkedList class represents a linked list. In the add method, the new node will be added to the end of the linked list.

Problem Solution:
To solve the problem of linked list loops, we need to check whether there is a loop in the linked list and find the location of the loop. This can be achieved using fast and slow pointers.

The basic idea of ​​the fast and slow pointer method is to place two pointers at the head of the linked list, one pointer moves one step at a time, and the other pointer moves two steps at a time. If there is a cycle in the linked list, the fast pointer will eventually catch up with the slow pointer.

Let’s modify the code of the LinkedList class and add a hasCycle method to check whether there is a cycle in the linked list:

public class LinkedList {
    Node head;
    
    public boolean hasCycle() {
        Node fast = head;
        Node slow = head;
        
        while(fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
            
            if(fast == slow) {
                return true;
            }
        }
        
        return false;
    }
}

in hasCycle In the method, we use fast and slow pointers to detect whether there is a cycle in the linked list. If there is a cycle, the fast and slow pointers will eventually meet, indicating that there is a cycle in the linked list.

Next, we modify the add method and add a logic to determine whether there is a cycle in the linked list:

public void add(int data) {
    Node newNode = new Node(data);
    
    if(hasCycle()) {
        throw new IllegalArgumentException("链表中存在循环");
    }
    
    if(head == null) {
        head = newNode;
    } else {
        Node current = head;
        
        while(current.next != null) {
            current = current.next;
        }
        
        current.next = newNode;
    }
}

Before adding a new node, we first call hasCycle Method to detect whether there is a cycle in the linked list. If a loop exists, throw an exception and alert the user.

Conclusion:
By using the method of fast and slow pointers, we can effectively detect whether there are cycles in the linked list and avoid cycles when adding nodes. This article provides a simple code example, hoping to help readers better understand and solve the problem of linked list loops. Of course, in actual development, we also need to debug and optimize according to specific circumstances to ensure the correctness and performance of the code.

References:

• [https://leetcode.com/problems/linked-list-cycle/](https://leetcode.com/problems/linked-list -cycle/)

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