本日耗時:1hr 26mins
因為最後實做出來不管怎麼改都Time Limit Exceeded
最終還是看答案了;)

題目敘述:
141. Linked List Cycle
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.

/**
 * 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) {
        if (head == nullptr || head->next == nullptr) {
            return false;
        }

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

        while (fast != nullptr && fast->next != nullptr) {
            slow = slow->next;
            fast = fast->next->next;

            if (slow == fast) {
                return true;  // 龜兔相遇，表示有循環
            }
        }

        return false;  // 龜兔沒有相遇，表示無循環
    }
};