LeetCode 142. Linked List Cycle II

https://leetcode.com/problems/linked-list-cycle-ii/

	/**
	* 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) {
	if(!head) return NULL;
	auto slow = head;
	auto fast = head;
	// Two Pointer
	// meet: slow & fast
	while(fast and fast->next){
	slow = slow->next;
	fast = fast->next->next;
	if(slow == fast) break;
	}
	// not cycle
	if(!fast or !fast->next) return NULL;
	// meet: head & slow
	while(head != slow){
	slow = slow->next;
	head = head->next;
	}
	return head;
	}
	};

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