剑指 Offer 链表

06. 从尾到头打印链表

class Solution {
public:
//两个指针一起走 一次翻转一个方向 最后head.next =null
ListNode* reverse1(ListNode *head){
    if(!head)return NULL;
  auto a=head,b=head->next;
  while(b){
      auto c=b->next;
      b->next=a;//只反转一个方向

      //指针往前走
      a=b,b=c;
  }
  head->next=NULL;
  return a;
}
    vector<int> reversePrint(ListNode* head) {
        head=reverse1(head);
        vector<int>a;
        while(head){
            a.push_back(head->val);
            head=head->next;
        }
        return a;
    }
};

24. 反转链表

class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if(!head)return head;
        if(!head->next)return head;
        ListNode* tail =reverseList(head->next);
        head->next->next=head;
        head->next=NULL;
        return tail;

    }
};