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  1. #include <iostream>
  2.  
  3. using namespace std;
  4.  
  5. class Node {
  6. public:
  7.     Node* next;
  8.     int id;
  9.     Node(int id, Node* next) : id(id), next(next) {}
  10.  
  11. };
  12.  
  13. Node* FindLoopBegin(Node *head) {
  14.     Node *slowptr = head,*fastptr = head;
  15.     bool LoopExists = false;
  16.     while(slowptr && fastptr){
  17.         fastptr = fastptr->next;
  18.         //if(fastptr == slowptr) {LoopExists = true;break;}
  19.         if(fastptr == NULL) {LoopExists = false; return NULL;}
  20.         fastptr = fastptr->next;
  21.         slowptr = slowptr->next;
  22.         if(fastptr == slowptr) {LoopExists = true;break;}
  23.     }
  24.     if(LoopExists) {
  25.         slowptr = head;
  26.         while(slowptr != fastptr){
  27.             slowptr = slowptr->next;
  28.             fastptr = fastptr->next;
  29.         }
  30.         return slowptr;
  31.     }   
  32.     return NULL;
  33. }
  34.  
  35. int main()
  36. {
  37.    Node* n10 = new Node(10,NULL);
  38.    Node* n12 = new Node(12,NULL);
  39.    Node* n14 = new Node(14,NULL);
  40.    n12->next = n14;
  41.    n14->next = n12;
  42.    n10->next = n12;
  43.    cout<<(FindLoopBegin(n10)->id)<<endl;
  44.    return 0;
  45. }
Success #stdin #stdout 0s 3272KB
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https://ideone.com/7Zy0tk
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
public

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