141. Linked List Cycle

快慢指针，当快指针和慢指针重叠时说明有cycle，while条件是为了没有cycle的时候跳出循环。

public boolean hasCycle(ListNode head) {
    ListNode fast = head, slow = head;
    while (fast != null && fast.next != null) { // in case that no cycle in the linked list
        fast = fast.next.next;
        slow = slow.next;
        if (fast == slow) {
            return true;
        }
    }
    return false;
}