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)
				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;
	}
};

更新I的O(n)算法：

//2014-2-19 update
	bool hasCycle(ListNode *head) 
	{
		ListNode *fast = head;
		ListNode *slow = head;
		while (fast)
		{
			slow = slow->next;
			fast = fast->next;
			if (fast) fast = fast->next;
			if (fast && slow == fast) return true;
		}
		return false;
	}