---
title: 'LeetCode 141. Linked List Cycle'
disqus: hackmd
---

# LeetCode 141. Linked List Cycle

## Description

Given head, the head of a linked list, determine if the linked list has a cycle in it.

There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. Internally, pos is used to denote the index of the node that tail's next pointer is connected to. Note that pos is not passed as a parameter.

Return true if there is a cycle in the linked list. Otherwise, return false.

## Example


Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where the tail connects to the 1st node (0-indexed).

## Constraints

The number of the nodes in the list is in the range [0, 10^4^].
-10^5^ <= Node.val <= 10^5^
pos is -1 or a valid index in the linked-list.

## Answer

此題可抓兩開頭點，一個一次走一步，另一個一次走兩步，若有cycle則此兩點一定會相遇。

//2022_03_12
bool hasCycle(struct ListNode *head){
    struct ListNode *fast = head, *slow = head;
    while(fast != NULL && fast->next != NULL){
        fast = fast->next->next;
        slow = slow->next;
        if(fast == slow){return true;}
    }
    return false;
}

## Link
https://leetcode.com/problems/linked-list-cycle/
###### tags: `Leetcode`