Linked List Cycle

Problem

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

Follow up:

Can you solve it without using extra space?

Solution

public class Solution {
    public boolean hasCycle(ListNode head) {
        if (head == null || head.next == null) return false;
        ListNode fast = head, slow = head;
        while (fast.next != null && fast.next.next != null) {
            fast = fast.next.next;
            slow = slow.next;
            if (fast == slow) return true;
        }
        return false;
    }
}

Analysis

Typical way to determine whether there is a cycle in a Linked List
We have two pointers fast and slow and we update that with difference speeds(metaphor)
If there is a cycle, they will meet eventually
This algorithm with finding the start node in Linked List is called Floyd's cycle finding algorithm