C++: Linked List Cycle

Thought Process

To detect a cycle in a linked list, we use the Floyd's Tortoise and Hare algorithm. This involves two pointers: a slow pointer that moves one step at a time and a fast pointer that moves two steps at a time. If there is a cycle, the fast pointer will eventually meet the slow pointer. If the fast pointer reaches the end of the list (NULL), it means there is no cycle. 

/**
 * 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 *slow = head;
        ListNode *fast = head;

        while(fast!=NULL && fast->next != NULL){

            slow = slow->next;
            fast = fast->next->next;

            if(fast==slow)
                return true;
        }
        return false;
    }
};

Code Complexity

Time Complexity: O(n)

The algorithm iterates through the linked list once, making it linear in time complexity.

Space Complexity: O(1)

The algorithm uses only two pointers, ensuring constant space usage.

Code copied!