tsanmmmm2000's Tech Blog

Question

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

Follow up:
Can you solve it without using extra space?

Thinking

要判斷一Linked List是否有循環存在，可以使用弗洛伊德演算法(Floyd Cycle Detection Algorithm)，亦被稱為龜兔賽跑算法(Tortoise and Hare Algorithm)，即假設烏龜與兔子以不同速率前進，若這條路存在循環，則兩者必定會有相遇的時刻，在Linked List上的概念亦是如此，如下圖

My C# Solution

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public bool HasCycle(ListNode head) {
        if (head == null || head.next == null) return false;
        
        var tortoise = head;
        var hare = head.next;
        while(tortoise != hare)
        {
            if (hare == null || hare.next == null) return false;
            tortoise = tortoise.next;
            hare = hare.next.next;
        }
        return true;
    }
}

Reference：Floyd判圈算法 - Wiki、Cycle Finding: Floyd's Algorithm