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Java-based LeetCode algorithm problem solutions, regularly updated

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package g0101_0200.s0141_linked_list_cycle;

// #Easy #Top_100_Liked_Questions #Top_Interview_Questions #Hash_Table #Two_Pointers #Linked_List
// #Data_Structure_I_Day_7_Linked_List #Udemy_Linked_List #Big_O_Time_O(N)_Space_O(1)
// #2022_06_24_Time_0_ms_(100.00%)_Space_45.5_MB_(68.52%)

import com_github_leetcode.ListNode;

/*
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
/**
 * 141 - Linked List Cycle\.
 *
 * Easy
 *
 * 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`.
 *
 * **Example 1:**
 *
 * ![](https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist.png)
 *
 * **Input:** head = [3,2,0,-4], pos = 1
 *
 * **Output:** true
 *
 * **Explanation:** There is a cycle in the linked list, where the tail connects to the 1st node (0-indexed). 
 *
 * **Example 2:**
 *
 * ![](https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test2.png)
 *
 * **Input:** head = [1,2], pos = 0
 *
 * **Output:** true
 *
 * **Explanation:** There is a cycle in the linked list, where the tail connects to the 0th node. 
 *
 * **Example 3:**
 *
 * ![](https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test3.png)
 *
 * **Input:** head = [1], pos = -1
 *
 * **Output:** false
 *
 * **Explanation:** There is no cycle in the linked list. 
 *
 * **Constraints:**
 *
 * *   The number of the nodes in the list is in the range [0, 10⁴].
 * *   -10⁵ <= Node.val <= 10⁵
 * *   `pos` is `-1` or a **valid index** in the linked-list.
 *
 * **Follow up:** Can you solve it using `O(1)` (i.e. constant) memory?
**/
public class Solution {
    public boolean hasCycle(ListNode head) {
        if (head == null) {
            return false;
        }
        ListNode fast = head.next;
        ListNode slow = head;
        while (fast != null && fast.next != null) {
            if (fast == slow) {
                return true;
            }
            fast = fast.next.next;
            slow = slow.next;
        }
        return false;
    }
}