Linked List Cycle

Problem

Given head, the head of a linked list, determine if the linked list has a cycle in it.

There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. Internally, pos is used to denote the index of the node that tail’s next pointer is connected to. Note that pos is not passed as a parameter.

Return true if there is a cycle in the linked list. Otherwise, return false.

Follow up: Can you solve it using O(1) (i.e. constant) memory?

Constraints

• The number of the nodes in the list is in the range [0, 104].
• -105 <= Node.val <= 105
• pos is -1 or a valid index in the linked-list.

Solution

The problem Linked List Cycle can be solved using Floyd’s cycle-finding algorithm. According to Floyd’s cycle-finding algorithm, if a fast pointer moving at twice the speed of another pointer meets with the slow pointer without reaching the end of the linked list, then there’s a cycle in the linked list.

Implementation

/**
 * 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)
    {
        ListNode *tortoise = head;
        ListNode *hare = head;

        while (hare != NULL && hare->next != NULL)
        {
            tortoise = tortoise->next;
            hare = hare->next->next;

            if (tortoise == hare)
                return true;
        }

        return false;
    }
};