141. Linked List Cycle

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.

 

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

'''
[1] slow and fast

idea::
slow pointer move one by one
fast pointer move two by two

if they meet, then has a cycle
else doesn't have a cycle

code::
def hasCycle(self, head: Optional[ListNode]) -> bool:
        slow,fast=head,head
        
        while fast!=None and fast.next!=None:
            fast=fast.next.next
            slow=slow.next
            
            if fast==slow:
                return True
        return False
        
-T/C: O(n)
-S/C: O(1)
'''
class Solution:
    def hasCycle(self, head: Optional[ListNode]) -> bool:
        slow,fast=head,head
        
        while fast!=None and fast.next!=None:
            fast=fast.next.next
            slow=slow.next
            
            if fast==slow:
                return True
        return False