LeetCode Linked List Cycle 解答程序

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?

错误解法。因为Cycle可能出现在Link的中间的，所以需要检查其中间Nodes。

bool hasCycle(ListNode *head) {

		// IMPORTANT: Please reset any member data you declared, as

		// the same Solution instance will be reused for each test case.

		if(head == NULL)

			return false;



		ListNode *cur = head;

		while(cur != NULL)

		{

			cur = cur->next;

			if(cur == head)

				return true;

		}

		return false;

	}

正确解法：

class Solution {

public:



	bool find(ListNode *head, ListNode *testpNode)

	{

		ListNode *p = head;

		while (p != testpNode->next)

		{

			if(p == testpNode && p != testpNode->next)

				return false;

			p = p->next;

		}

		return true;

	}





	bool hasCycle(ListNode *head) {

		// IMPORTANT: Please reset any member data you declared, as

		// the same Solution instance will be reused for each test case.

		if(head == NULL)

			return false;



		ListNode *cur = head;

		while(cur != NULL)

		{

			if(find(head, cur))

				return true;

			cur = cur->next;

		}

		return false;

	}

};