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public class Floyd {

/**

* Floyd判圈算法（双指针）

* @param head

* @return

*/

public boolean hasCycle(ListNode head){

if(head == null || head.next == null)

return false;

ListNode slow = head;

ListNode fast = head;

do{

if(fast == null || fast.next == null) return false;

slow = slow.next;

fast = fast.next.next;

}while(slow != fast);

return true;

}

/**

* 如果存在环，则求出环长

* @param head

* @return

*/

public int countCycleLength(ListNode head) {

if(!hasCycle(head)){

return 0;

}

ListNode slow = head;

ListNode fast = head;

do{

slow = slow.next;

fast = fast.next.next;

}while(slow != fast);

int cycleLength = 0;

do{

slow = slow.next;

fast = fast.next.next;

cycleLength++;

}while(slow != fast);

return cycleLength;

}

//找到环的起始位置

public ListNode detectCycle(ListNode head) {

if(!hasCycle(head)){

return null;

}

ListNode slow = head;

ListNode fast = head;

do{

slow = slow.next;

fast = fast.next.next;

}while(slow != fast);

ListNode start = head;

while(slow != start){

slow = slow.next;

start = start.next;

}

return start;

}

}