141.Linked List Cycle

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Solution 1: accepted

The use of dummy is optional. If we do fast = head, we need to verify that head != null.

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public boolean hasCycle(ListNode head) {
    ListNode dummy = new ListNode(1);
    dummy.next = head;
    ListNode fast = dummy;
    ListNode slow = dummy;
    while (fast.next != null && fast.next.next != null) {
        fast = fast.next.next;
        slow = slow.next;
        if (fast == slow) {
            return true;
        }
    }
    return false;
}