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?

划重点 o(1)很自然想到用哈希表来解决

public static boolean hasCycle(ListNode head) {
        Set<ListNode> set = new HashSet<>();
        while (head != null) {
            if (set.contains(head)) {  //set中已经有，说明有环
                return true;
            } else {
                set.add(head);
            }
            head = head.next;
        }
        return false;
    }