# 141. Linked List Cycle
Given a linked list, determine if it has a cycle in it.
To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.
Example 1:
Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the second node.
Example 2:
Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the first node.
Example 3:
Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.
Follow up:
Can you solve it using O(1) (i.e. constant) memory?
# Solution
Approach 1: Hash table.
Approach 2: Two pointers.
# Code (Python)
Approach 1:
Approach 2:
# Code (C++)
Approach 1:
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
bool hasCycle(ListNode *head) {
unordered_set<ListNode*> nodeSet;
for (ListNode *p = head; p != NULL; p = p->next)
{
if (nodeSet.find(p) != nodeSet.end())
return true;
else
nodeSet.insert(p);
}
return false;
}
};
Approach 2:
class Solution {
public:
bool hasCycle(ListNode *head) {
ListNode *slow = head;
ListNode *fast = head;
while (fast != NULL && fast->next != NULL)
{
slow = slow->next;
fast = fast->next->next;
if (slow == fast)
return true;
}
return false;
}
};