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  • 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.
    

     

    Follow up:

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

    空间复杂度O(n)

    public boolean hasCycle(ListNode head) {//链表 
            Set<ListNode> nodeSet = new HashSet<ListNode>();
            while(null!= head){
                if(nodeSet.contains(head)){
                    return true;
                }
                else{
                    nodeSet.add(head);
                    head=head.next;
                }
            }
            return false;
        }  

    空间复杂度O(1)

    设置快慢两个指针,若有环,两个指针定会相遇

     public boolean hasCycle(ListNode head) {
            ListNode oneStep = head;
            ListNode twoStep  =head;
            while(null!=oneStep&&null!=twoStep&&null!=twoStep.next){
                oneStep=oneStep.next;
                twoStep=twoStep.next.next;
                if (oneStep==twoStep){
                    return true;
                }
            }
            return false;
        }  

    进阶题

    查询有环链表的环入口 LeetCode142 https://www.cnblogs.com/zhacai/p/10561152.html

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  • 原文地址:https://www.cnblogs.com/zhacai/p/10560803.html
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