LeetCode:Linked List Cycle

Given a linked list, determine if it has a cycle in it.

Follow up:

Can you solve it without using extra space?

bool hasCycle(ListNode *head)
{

<span style="white-space:pre">	</span>if(head == NULL)
<span style="white-space:pre">		</span>return false;

<span style="white-space:pre">	</span>ListNode* pslow = head;
<span style="white-space:pre">	</span>ListNode* pfast = head;

<span style="white-space:pre">	</span>while(pfast != NULL && pfast ->next != NULL)
<span style="white-space:pre">	</span>{
<span style="white-space:pre">		</span>pslow = pslow->next;
<span style="white-space:pre">		</span>pfast = pfast->next->next;
<span style="white-space:pre">		</span>if(pfast == pslow)
<span style="white-space:pre">			</span>return true;
<span style="white-space:pre">	</span>}
<span style="white-space:pre">	</span>return false;
}

分析：用到了快慢指针。如果快指针追上了慢指针，说明存在循环，否则没有。

已AC 76ms