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Given a linked list, determine if it has a cycle in it.
Follow up:
Can you solve it without using extra space?
给定一个链表，确定它是否有一个循环。 你能在不使用额外空间的情况下解决它吗?
//两个速度指针
public boolean hasCycle(ListNode head) {
if(head ==null){
return false;
}
ListNode walker = head;
ListNode runner = head;
while(runner!=null && runner.next!=null){
walker = walker.next;
runner = runner.next.next;
if(walker == runner){
return true;
}
}
return false;
}