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public class LinkedListCycleII {

/**

* 这里有个问题，如果链表是a,b，然后又回到a

* 则slow和fast必须都从head起步才行

* 如果slow从head起步，fast从head.next起步，则这里会死循环

*/

public ListNode detectCycle(ListNode head) {

ListNode slow = head, fast = head;

boolean hasCycle = false;

for (; fast != null && fast.next != null; ) {

slow = slow.next;

fast = fast.next.next;

if (slow == fast) {

hasCycle = true;

break;

}

}

if (!hasCycle) {

return null;

}

for (ListNode node = head; node != slow; node = node.next, slow = slow.next);

return slow;

}

}