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LinkedListCycleII.cpp
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LinkedListCycleII.cpp
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/*
Given a linked list, return the node where the cycle begins. If there is no cycle, return null.
Follow up:
Can you solve it without using extra space?
Written by Xi Wang
11/15/2013
*/
/**
* 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) {
// IMPORTANT: Please reset any member data you declared, as
// the same Solution instance will be reused for each test case.
if(!head)
return NULL;
ListNode *slow = head;
ListNode *fast = head;
while(fast&&fast->next)
{
slow = slow->next;
fast = fast->next->next;
if(slow==fast)
break;
}
if(!fast ||!fast->next)
return NULL;
// find the start of the cycle begin
fast = head;
while(fast!=slow)
{
fast = fast->next;
slow = slow->next;
}
return fast;
}
};