使用 Map 检测链表中的循环

给定一个链表，检查链表是否有循环。
这里显示了各种方法：检测链表中的循环

例子

Input: 20->4->54->6->NULL 
Output: No loop is detected. 
Explanation: 
While traversing the linked list, we reach the end of the linked list. Therefore, no loop is present in the linked list.

Input: 20->4->5->10->20 
Output: Loop detected. 
Explanation: 
While traversing the linked list, reaching the node with value 10, it is linked with the head node, which depicts a loop in the linked list. Therefore, a loop is present in the linked list. 

方法：

1. 创建一个映射，将访问的节点存储在链表中。
2. 遍历链表并执行以下操作：
   • 检查当前节点是否存在于地图上。
   • 如果当前节点不存在于地图中，则将当前节点插入到地图中。
   • 如果节点存在于地图中，则检测到链表中的循环。
3. 如果我们在遍历链表时到达空节点，则给定的链表中没有循环。

下面是上述方法的实现：

C++

// C++ program to detect loop in
// given linked list using map
#include 
using namespace std;
 
// Structure for a node in Linked List
struct Node {
    int data;
    Node* next;
};
 
// Function to create Linked List
// Node
Node* newNode(int d)
{
    Node* temp = new Node;
    temp->data = d;
    temp->next = NULL;
    return temp;
}
 
// Declaration of Map to keep
// mark of visited Node
map vis;
bool flag = 0;
 
// Function to check cycle in Linked
// List
void check(Node* head)
{
    // If head is NULL return ;
    if (head == NULL) {
        flag = 0;
        return;
    }
 
    // Mark the incoming Node as
    // visited if it is not visited yet
    if (!vis[head]) {
        vis[head] = true;
        check(head->next);
    }
 
    // If a visited Node is found
    // Update the flag value to 1
    // and return ;
    else {
        flag = 1;
        return;
    }
}
 
// Driver Code
int main()
{
    // Create a head Node
    Node* head = newNode(20);
 
    // Inserting Node in Linked List
    head->next = newNode(4);
    head->next->next = newNode(5);
    head->next->next->next = newNode(10);
 
    // Just to make a cycle
    head->next->next->next->next = head;
 
    // Function that detect cycle in
    // Linked List
    check(head);
 
    // If flag is true, loop is found
    if (flag)
        cout << "Loop detected.";
 
    // If flag is false, No Loop
    // detected
    else
        cout << "No Loop Found.";
    cout << endl;
 
    return 0;
}

Java

// Java program to detect loop in
// given linked list using map
import java.util.*;
 
class GFG{
 
// Structure for a node in Linked List
static class Node
{
    int data;
    Node next;
};
 
// Function to create Linked List
// Node
static Node newNode(int d)
{
    Node temp = new Node();
    temp.data = d;
    temp.next = null;
    return temp;
}
 
// Declaration of Map to keep
// mark of visited Node
static HashMap vis = new HashMap<>();
static boolean flag = false;
 
// Function to check cycle in Linked
// List
static void check(Node head)
{
     
    // If head is null return ;
    if (head == null)
    {
        flag = false;
        return;
    }
 
    // Mark the incoming Node as
    // visited if it is not visited yet
    if (vis.containsKey(head))
    {
        vis.put(head, true);
        check(head.next);
    }
 
    // If a visited Node is found
    // Update the flag value to 1
    // and return ;
    else
    {
        flag = true;
        return;
    }
}
 
// Driver Code
public static void main(String[] args)
{
     
    // Create a head Node
    Node head = newNode(20);
 
    // Inserting Node in Linked List
    head.next = newNode(4);
    head.next.next = newNode(5);
    head.next.next.next = newNode(10);
 
    // Just to make a cycle
    head.next.next.next.next = head;
 
    // Function that detect cycle in
    // Linked List
    check(head);
 
    // If flag is true, loop is found
    if (flag)
        System.out.print("Loop detected.");
 
    // If flag is false, No Loop
    // detected
    else
        System.out.print("No Loop Found.");
         
    System.out.println();
}
}
 
// This code is contributed by Rajput-Ji

C#

// C# program to detect loop in
// given linked list using map
using System;
using System.Collections.Generic;
 
class GFG{
 
// Structure for a node in Linked List
public class Node
{
    public int data;
    public Node next;
};
 
// Function to create Linked List
// Node
static Node newNode(int d)
{
    Node temp = new Node();
    temp.data = d;
    temp.next = null;
    return temp;
}
 
// Declaration of Map to keep
// mark of visited Node
static Dictionary vis = new Dictionary();
static bool flag = false;
 
// Function to check cycle in Linked
// List
static void check(Node head)
{
     
    // If head is null return ;
    if (head == null)
    {
        flag = false;
        return;
    }
 
    // Mark the incoming Node as
    // visited if it is not visited yet
    if (vis.ContainsKey(head))
    {
        vis.Add(head, true);
        check(head.next);
    }
 
    // If a visited Node is found
    // Update the flag value to 1
    // and return ;
    else
    {
        flag = true;
        return;
    }
}
 
// Driver Code
public static void Main(String[] args)
{
     
    // Create a head Node
    Node head = newNode(20);
     
    // Inserting Node in Linked List
    head.next = newNode(4);
    head.next.next = newNode(5);
    head.next.next.next = newNode(10);
     
    // Just to make a cycle
    head.next.next.next.next = head;
 
    // Function that detect cycle in
    // Linked List
    check(head);
 
    // If flag is true, loop is found
    if (flag)
        Console.Write("Loop detected.");
 
    // If flag is false, No Loop
    // detected
    else
        Console.Write("No Loop Found.");
         
    Console.WriteLine();
}
}
 
// This code is contributed by gauravrajput1

Javascript

// javascript program to detect loop in
// given linked list using map
// Structure for a node in Linked List
    class Node {
        constructor(val) {
            this.data = val;
            this.next = null;
        }
    }
 
 
    // Function to create Linked List
    // Node
    function newNode(d) {
        var temp = new Node();
        temp.data = d;
        temp.next = null;
        return temp;
    }
 
    // Declaration of Map to keep
    // mark of visited Node
    var vis = new Map();;
    var flag = false;
 
    // Function to check cycle in Linked
    // List
    function check(head) {
 
        // If head is null return ;
        if (head == null) {
            flag = false;
            return;
        }
 
        // Mark the incoming Node as
        // visited if it is not visited yet
        if (!vis.has(head)) {
            vis.set(head, true);
            check(head.next);
        }
 
        // If a visited Node is found
        // Update the flag value to 1
        // and return ;
        else {
            flag = true;
            return;
        }
    }
 
    // Driver Code
     
 
        // Create a head Node
        var head = newNode(20);
 
        // Inserting Node in Linked List
        head.next = newNode(4);
        head.next.next = newNode(5);
        head.next.next.next = newNode(10);
 
        // Just to make a cycle
        head.next.next.next.next = head;
 
        // Function that detect cycle in
        // Linked List
        check(head);
 
        // If flag is true, loop is found
        if (flag)
            document.write("Loop detected.");
 
        // If flag is false, No Loop
        // detected
        else
            document.write("No Loop Found.");
 
        document.write();
 
// This code contributed by Rajput-Ji

输出：

Loop detected.

时间复杂度： O(N*log N)
辅助空间： O(N)

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