Linked List Cycle

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

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

Ideas:

1. You can use the hash table to record the address of each node in the list, and then iterate through the discovery of duplicates, that is, there is a loop, otherwise reach the end of no ring

2. Space complexity constant, two pointers, one step, one walk two steps, fast will catch up slow.

Code

classSolution { Public:    BOOLHascycle (ListNode *head) {        if(Head==null)return false; if(Head->next==null)return false; if(Head->next==head)return true; //at least 2 nodes.listnode* pone=Head; ListNode* ptwo=Head;  Do{            if(ptwo->next==NULL)return false; Ptwo=ptwo->next->Next; Pone=pone->Next; if(ptwo==NULL)return false; }         while(pone!=ptwo); return true; }};

Linked List Cycle