LeetCode Q142: 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?

Analysis:

Please refer to this post

	/**
	* 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) {
	ListNode* slow=head;
	ListNode* fast=head;
	while(slow!=NULL&&fast!=NULL){
	slow=slow->next;
	fast=fast->next;
	if(fast==NULL)
	return NULL;
	else
	fast=fast->next;
	if(slow==fast)
	break;
	}
	if(slow==NULL||fast==NULL)
	return NULL;
	slow=head;
	while(slow!=fast){
	slow=slow->next;
	fast=fast->next;
	}
	return slow;
	}
	};

view raw Q142CircleListII.cpp hosted with ❤ by GitHub