去Twitter面试的被问到这个问题,当时只想到了用HashMap的办法,这种办法时间复杂度O(n),空间复杂度是O(n), 更好的办法是用 FastRunner / SlowRunner approach。用两个pointer遍历链表,fast的速度是slow的两倍,如果有loop,二者一定会collide。

boolean detectLoop(LinkedListNode head){
LinkedList slow = head;
LinkedList fast = head;
while(fast != null && fast.next != null){
slow = slow.next;
fast = fast.next.next;
     if(slow == fast){
return true;
}
}
return false;
}

What if we want to find the start of the loop?

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