141. Linked List Cycle

Problem Description

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.

Examples

Example 1:
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).

Example 2:
Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where the tail connects to the 0th node.

Example 3:
Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.

def hasCycle(head: Optional[ListNode]) -> bool:
    if not head or not head.next:
        return False
    
    slow = head
    fast = head.next
    
    while slow != fast:
        if not fast or not fast.next:
            return False
        slow = slow.next
        fast = fast.next.next
    
    return True

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

class Solution {
public:
    bool hasCycle(ListNode *head) {
        if (!head || !head->next) {
            return false;
        }
        
        ListNode *slow = head;
        ListNode *fast = head->next;
        
        while (slow != fast) {
            if (!fast || !fast->next) {
                return false;
            }
            slow = slow->next;
            fast = fast->next->next;
        }
        
        return true;
    }
};

/**
 * @param {ListNode} head
 * @return {boolean}
 */
var hasCycle = function(head) {
    if (!head || !head.next) {
        return false;
    }
    
    let slow = head;
    let fast = head.next;
    
    while (slow !== fast) {
        if (!fast || !fast.next) {
            return false;
        }
        slow = slow.next;
        fast = fast.next.next;
    }
    
    return true;
};

public class Solution {
    public bool HasCycle(ListNode head) {
        if (head == null || head.next == null) {
            return false;
        }
        
        ListNode slow = head;
        ListNode fast = head.next;
        
        while (slow != fast) {
            if (fast == null || fast.next == null) {
                return false;
            }
            slow = slow.next;
            fast = fast.next.next;
        }
        
        return true;
    }
}