Leetcode 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.

这道题让我们判断一个链表是否有环，可以通过快慢指针来判断，这里要注意的就是循环的终止条件，因为快指针每次走两步，因此要判断快指针是否为空以及快指针的下一个节点是否为空。

C++
bool hasCycle(ListNode *head) {
        if(head == nullptr)
            return false;
        ListNode* slow = head;
        ListNode* fast = slow->next;
        while(fast != nullptr&&fast->next != nullptr)
        {
            if(fast == slow)
                return true;
            fast = fast->next->next;
            slow = slow->next;
        }
        return false;
    }