LeetCode-linked-list-cycle-ii

题目：如果一个链表循环，返回第一个进入循环的点，否则返回NULL。

class Solution {public:    ListNode *detectCycle(ListNode *head) {        if(head==NULL || head->next==NULL)              return NULL;          ListNode* pslow = head;          ListNode* pfast = head;          while(pfast!=NULL)          {              if(pfast != NULL)                  pfast = pfast->next;              if(pfast != NULL)                  pfast = pfast->next;              if(pfast==NULL)                  break;              pslow = pslow->next;            if(pfast != NULL && pslow == pfast){                pslow = head;                while(pslow != pfast){                    pslow = pslow->next;                    pfast = pfast->next;                }                return pslow;            }          }        return NULL;            }};