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LeetCode: Linked List Cycle II

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/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode *detectCycle(ListNode *head) {
auto end = findEnd(head);
if(end==NULL)
return NULL;
auto lhs=head, rhs=end->next;
int llen=length(lhs, end), rlen=length(rhs, end);
if(llen<rlen)
{
swap(llen, rlen);
swap(lhs, rhs);
}
while(llen>rlen)
{
lhs=lhs->next;
--llen;
}

while(lhs!=rhs)
{
lhs=lhs->next;
rhs=rhs->next;
}
return lhs;
}

private:
ListNode *findEnd(ListNode *head)
{
if(head==NULL || head->next==NULL)
return NULL;
auto slow=head, fast=head->next;
while(slow && fast)
{
if(slow==fast)
return slow;
slow=slow->next;
if(fast->next)
fast=fast->next->next;
else
return NULL;
}
return NULL;
}

int length(ListNode *head, ListNode *tail)
{
if(head==NULL)
return 0;
if(head==tail)
return 1;
else return 1+length(head->next, tail);
}
};