**Linked Lists: Detect a Cycle**

### Problem Statement :

A linked list is said to contain a cycle if any node is visited more than once while traversing the list. For example, in the following graph there is a cycle formed when node 5 points back to node 3. Function Description Complete the function has_cycle in the editor below. It must return a boolean true if the graph contains a cycle, or false. has_cycle has the following parameter(s): head: a pointer to a Node object that points to the head of a linked list. Returns boolean: True if there is a cycle, False if there is not Note: If the list is empty, head will be null. Input Format There is no input for this challenge. A random linked list is generated at runtime and passed to your function. Constraints 0 <= list size <= 100

### Solution :

` ````
Solution in C++ :
In C++ :
/*
A Node is defined as:
struct Node {
int data;
Node* next;
}
*/
#include <unordered_set>
std::unordered_set<Node*> visited;
bool has_cycle(Node* head)
{
// Empty lists don't have circles.
if(head == nullptr) {
visited.clear();
return false;
}
// If already in the list of visited nodes.
if(visited.count(head) != 0) {
visited.clear();
// It means we have a cycle.
return true;
}
// Otherwise, remember it.
visited.insert(head);
// Recurse for the rest of the list.
return has_cycle(head->next);
}
```

` ````
Solution in Java :
In Java :
/*
A Node is defined as:
class Node {
int data;
Node next;
}
*/
boolean hasCycle(Node head) {
if(head==null)
return false;
else{
Node slow=head;
Node fast=head.next;
while(fast!=null && fast.next!=null && fast!=slow){
slow=slow.next;
fast=fast.next.next;
}
if( fast!=null && fast==slow)
return true;
return false;
}
}
```

` ````
Solution in Python :
In Python3 :
"""
A Node is defined as:
class Node(object):
def __init__(self, data = None, next_node = None):
self.data = data
self.next = next_node
"""
def has_cycle(head):
visited = set()
it = head
while it.next:
it = it.next
if it in visited:
return True
visited.add(it)
return False
```

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