java 解法, 执行用时: 0 ms, 内存消耗: 42.6 MB, 提交时间: 2023-07-30 23:13:32

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode detectCycle(ListNode head) {
        ListNode slow = head;
        ListNode fast = head;
        while ( fast != null ) {
            slow = slow.next;
            if ( fast.next == null ) {
                return null;
            }
            fast = fast.next.next;
            if ( fast == slow ) {
                ListNode p = head;
                while ( p != slow ) {
                    p = p.next;
                    slow = slow.next;
                }
                return p;
            }
        }
        return null;
    }
}

python3 解法, 执行用时: 56 ms, 内存消耗: 18.5 MB, 提交时间: 2022-07-10 23:15:59

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def detectCycle(self, head: ListNode) -> ListNode:
        slow, fast = head, head
        while fast:
            slow = slow.next
            if fast.next is None:
                return None
            fast = fast.next.next
            if fast == slow:
                p = head
                while p != slow:
                    p = p.next
                    slow = slow.next
                return p
        return None

golang 解法, 执行用时: 4 ms, 内存消耗: 3.7 MB, 提交时间: 2021-08-10 10:23:46

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func detectCycle(head *ListNode) *ListNode {
    slow, fast := head, head
    for fast != nil {
        slow = slow.Next
        if fast.Next == nil {
            return nil
        }
        fast = fast.Next.Next
        if fast == slow {
            p := head
            for p != slow {
                p = p.Next
                slow = slow.Next
            }
            return p
        }
    }
    return nil
}