Java：给定一个链表，判断链表中是否有环。

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