[LeetCode] Problem 141 - Linked List Cycle

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

To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.

Example

No.1

Input: head = [3,2,0,-4], pos = 1

Output: true

Explanation: There is a cycle in the linked list, where tail connects to the second node.

AU5Gp4.png

No.2

Input: head = [1,2], pos = 0

Output: true

Explanation: There is a cycle in the linked list, where tail connects to the first node.

AU5Un1.png

No.3

Input: head = [1], pos = -1

Output: false

Explanation: There is no cycle in the linked list.

AU5DhD.png

Follow up

Can you solve it using O(1) (i.e. constant) memory?

Code

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public class ListNode {
    int val;
    ListNode next;
    ListNode(int x) {
        val = x;
        next = null;
    }
}
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public boolean hasCycle(ListNode head) {
    if (head == null)
        return false;
    
    ListNode fast = head;
    ListNode slow = head;

    while (fast.next != null && fast.next.next != null) {
        fast = fast.next.next;
        slow = slow.next;

        if (fast == slow)
            return true;
    }

    return false;
}