Linked List Cycle

ID:141

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.

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).

Idea

Slow+fast pointer

Since we don't know when the tail of the linked list happens, we cannot just iterate and find null since we don't know where it's suppose to terminate

General thought of assigning a slow moving pointer and a fast moving pointer, when fast moving one meets the slow moving one, meaning that the fast one have cycled over and meet the slow one, otherwise, there's no cycle

Code

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