[Leetcode]: 141:linked List Cycle

Topic:

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

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

Ideas:

Normal traversal, using hashset to determine whether to repeat

Code:

     Public Boolean hascycle (ListNode head) {        new  HashSet ();          while NULL ) {            if(!  Hashflag.add (head                ) {returntrue;            }             = head.next;        }         return false ;    }

Idea two: Using fast pointers and slow pointers

Two pointers, moving the speed difference one, so the fast pointer must be "catch up" slow pointer. It's sort of like a primary school chase.

Code:

     Public Booleanhascycle (ListNode head) {ListNode Fast=Head; ListNode Slow=Head;  while(slow!=NULL){            if(slow.next!=NULL&& fast.next!=NULL&& fast.next.next!=NULL) {Slow=Slow.next; Fast=Fast.next.next; if(Slow = =Fast) {                    return true; }            }            Else{                return false; }        }        return false; }

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[Leetcode]: 141:linked List Cycle