Interview Classic 150 Questions P141 Linked List Cycle

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Timeline

Timeline

2025-11-19

init

Linked list, two pointers

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Problem:

P141 Linked List Cycle

https://leetcode.cn/problems/linked-list-cycle/description/?envType=study-plan-v2&envId=top-interview-150

Two pointers, fast and slow pointers

struct ListNode {
	int val;
	ListNode *next;
	ListNode(int x)
		: val(x)
		, next(nullptr)
	{
	}
};

class Solution {
    public:
	bool hasCycle(ListNode *head)
	{
		ListNode *low = head, *fast = head;
		do {
			if (low == nullptr || low->next == nullptr)
				return false;
			else
				low = low->next;

			if (fast == nullptr || fast->next == nullptr || fast->next->next == nullptr)
				return false;
			else
				fast = fast->next->next;


		} while (low != fast);

		if (low != nullptr)
			return true;

		return false;
	}
};