leetcode 141.Linked List Cycle（链表环）

原创已于 2022-03-08 09:39:16 修改 · 101 阅读

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于 2019-05-04 13:11:43 首次发布

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博客围绕判断单向链表是否存在环展开。通过给定链表及表示尾连接位置的整数pos，若pos为 -1 则无环。采用快慢指针法，快慢指针初始都指向head，快指针每次走两步，慢指针每次走一步，若相遇则链表有环。

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.

问单向链表中是否存在环。

思路：
典型的快慢指针问题，刚开始快慢指针同时指向head, 然后快指针每次走两步，慢指针每次走一步，如果它们能相遇，则有环。

	public boolean hasCycle(ListNode head) {
        if(head == null) return false;
        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;
    }