leetcode每日一题2020/10/09

https://leetcode-cn.com/problems/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.

Follow up:

Can you solve it using O(1) (i.e. constant) memory?

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func hasCycle(head *ListNode) bool {
     
    seen := map[*ListNode]struct{
     }{
     }
    for head != nil {
     
        if _, ok := seen[head]; ok {
     
            return true
        }
        seen[head] = struct{
     }{
     }
        head = head.Next
    }
    return false
}

其实更好用双指针法