LeetCode/src/easy/LinkedListCycle.java at master · chenyiAlone/LeetCode · GitHub

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package easy;

import util.ListNode;
/**
 *
 * ClassName: LinkedListCycle
 * @author chenyiAlone
 * Create Time: 2019/01/05 13:06:31
 * Description: No.141
 * 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.

 */
public class LinkedListCycle {
    public boolean hasCycle(ListNode head) {
        ListNode slow = head;
        ListNode fast = head;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            if (slow != null && fast != null && slow.val == fast.val) {
                return true;
            }
        }
        return false;
    }
}