leetcode——142——Linked List Cycle II

Given a linked list, return the node where the cycle begins. If there is no cycle, return

null

.
Note: Do not modify the linked list.

Follow up:

Can you solve it without using extra space?

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
int numNode = 0;
ListNode *detectCycle(ListNode *head) {
if (!hasCycle(head))
return NULL;

ListNode* pBegin = head;
ListNode* pEnd = head;

for (int i = 0; i < numNode; i++)
pBegin = pBegin->next;

while (pBegin != pEnd)
{
pBegin = pBegin->next;
pEnd = pEnd->next;
}
return pBegin;

}
bool hasCycle(ListNode *head) {
if (head == NULL || head->next == NULL)
return false;

ListNode* fast = head;
ListNode* slow = head;

while (fast != NULL)
{
fast = fast->next;
if (fast != NULL)
{
fast = fast->next;
slow = slow->next;
numNode++;
}
if (fast == slow)
return true;

}
return false;

}

};