141.Linked List Cycle

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Linked List, Easy

Question

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

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

Answer

HashSet

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/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public boolean hasCycle(ListNode head) {
        HashSet<ListNode> hashset = new HashSet<>();
        while(head!=null){
            if(hashset.contains(head))
                return true;
            else{
                hashset.add(head);
                head = head.next;
            }
        }
        return false;
    }
}

Running time: O(n)

Fast and Slow pointer

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/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public boolean hasCycle(ListNode head) {
        ListNode fast = head;
        ListNode slow = head;
        
        while(fast!=null&&fast.next!=null){  // in case: [1] only one element
            fast = fast.next.next;
            slow = slow.next;
            if(fast == slow)
                return true;
        }
        return false;
    }
}

Running time: O(n)