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  1. #include <iostream>
  2. using namespace std;
  3.  
  4. struct ListNode {
  5.     int value;
  6.     ListNode* prev;
  7.     ListNode* next;
  8.     ListNode(int val) : value(val), prev(nullptr), next(nullptr) {}
  9. };
  10.  
  11. bool hasCycle(ListNode* head) {
  12.     ListNode* current = head;
  13.  
  14.     // Checking the base case whether DL is Circular DL or not
  15.     // If the DL is circular with just head->prev nullptr but tail->next = head : This case will be checked in while loop
  16.     if(current->prev != nullptr) return true;
  17.     while (current->next != nullptr) {
  18.         // If the previous node of the next node is not the current node itself,
  19.         // it indicates the presence of a loop
  20.         if (current->next->prev != current) {
  21.             return true;
  22.         }
  23.         current = current->next;
  24.     }
  25.      // If traversal completes without encountering a loop, we return false
  26.     return false;
  27. }
  28.  
  29. //function to print the doubly linked list
  30. void printList(ListNode* head) {
  31.     ListNode* current = head;
  32.     while (current != nullptr) {
  33.         cout << current->value << " ";
  34.         current = current->next;
  35.     }
  36.     cout <<endl;
  37. }
  38.  
  39. int main() {
  40.     ListNode* node1 = new ListNode(1);
  41.     ListNode* node2 = new ListNode(2);
  42.     ListNode* node3 = new ListNode(3);
  43.     ListNode* node4 = new ListNode(4);
  44.  
  45.     node1->next = node2;
  46.     node2->prev = node1;
  47.     node2->next = node3;
  48.     node3->prev = node2;
  49.     node3->next = node4;
  50.     node4->prev = node3;
  51.     node4->next = node2;  // Cycle back to node2
  52.  
  53.     cout << (hasCycle(node1) ? "list contain a cycle" : "list does not contain a cycle") <<endl;
  54.  
  55.     // Freeing memory
  56.     delete node1;
  57.     delete node2;
  58.     delete node3;
  59.     delete node4;
  60.  
  61.     return 0;
  62. }
  63.  
Success #stdin #stdout 0.01s 5236KB
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stdin
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Standard input is empty
stdout
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list contain a cycle
https://ideone.com/x3rcfE
language:
C++ 4.3.2 (gcc-4.3.2)
created:
29 days ago
visibility:
public

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