leetcode 206

1. Reverse Linked List
   Reverse a singly linked list.

1、swap in-placeListNode* reverseList(ListNode* head) {        if(head==NULL || head->next==NULL)             return head;          ListNode *pre = head;          ListNode *p = head->next;          pre->next = NULL;          ListNode *nxt;          while(p!=NULL) {              nxt = p->next;              p->next = pre;              pre = p;              p = nxt;          }        return pre;    }