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First_node_of_loop_in_list.cpp
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First_node_of_loop_in_list.cpp
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/*
Problem Statement:
------------------
Write a function that checks whether a given Linked List contains a loop. If the loop is present
then it returns point to the first node of the loop. Else it returns NULL.
Example:
-------
Input: head = [3,2,0,-4].
Output: 2
As:
3 --> 2 --> 0 --> -4
| |
------------
Explanation: There is a cycle in the linked list, where the tail connects to the 1st node (0-indexed). HEnce return 2.
*/
// Link --> https://www.geeksforgeeks.org/find-first-node-of-loop-in-a-linked-list/
// Code:
#include <bits/stdc++.h>
using namespace std;
struct Node
{
int data;
struct Node* next;
};
Node* newNode(int key)
{
Node* temp = new Node;
temp->data = key;
temp->next = NULL;
return temp;
}
/*
void printList(Node* head)
{
while (head != NULL)
{
cout<<head->data<<" ";
head = head->next;
}
cout<<endl;
}
*/
Node* detectFirstLoopNode(Node* head)
{
if(head == NULL || head->next == NULL)
return NULL;
Node *slow = head;
Node *fast = head->next;
while(fast != slow )
{
if(!fast || !slow)
return NULL;
fast = fast->next->next;
slow = slow->next;
}
return slow;
}
int main()
{
Node* head = newNode(50);
head->next = newNode(20);
head->next->next = newNode(15);
head->next->next->next = newNode(4);
head->next->next->next->next = newNode(10);
// A loop for testing.
head->next->next->next->next->next = head->next->next;
Node* answer = detectFirstLoopNode(head);
if(answer)
cout<<"First loop node is : "<<answer->data<<endl;
else
cout<<"No loop is present!"<<endl;
return 0;
}