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;
}
};