day3打卡

203 移除链表元素
/**

• Definition for singly-linked list.

• struct ListNode {

• int val;
  

• ListNode *next;
  

• ListNode() : val(0), next(nullptr) {}
  

• ListNode(int x) : val(x), next(nullptr) {}
  

• ListNode(int x, ListNode *next) : val(x), next(next) {}
  

• };
  /
  class Solution {
  public:
  ListNode removeElements(ListNode* head, int val) {
  ListNode *p = head;
  ListNode *q = NULL;
  if(head != nullptr)
  {
  q = p->next;
  }
  while(q != NULL)
  {
  if(q->val == val)
  {
  ListNode *tmp = q->next;
  p->next = q->next;

           q = tmp;
       }
       else
       {
           p = p->next;
           q= q->next;
       }
   }
   if(head != nullptr)
   {
       ListNode *temp = head->next;
       if(head->val == val)
       {
           head = temp;
       }
   }
   return head;
  

  }
  };
  设计链表
  class MyLinkedList {
  public:
  struct ListNode{
  int val;
  ListNode *next;
  ListNode(int val):val(val), next(nullptr){}
  };
  MyLinkedList()
  {
  _head = new ListNode(0);
  _size = 0;
  }

  int get(int index)
  {
  if(index > (_size - 1) || index < 0)
  {
  return -1;
  }
  ListNode *cur = _head->next;
  for(int i = 0; i < index; ++i)
  {
  cur = cur->next;
  }
  return cur->val;
  }

  void addAtHead(int val)
  {
  ListNode *tmp = new ListNode(val);
  if(_size == 0)
  {
  _head->next =tmp;
  }
  else
  {
  tmp->next = _head->next;
  _head->next = tmp;
  }
  _size++;
  }

  void addAtTail(int val)
  {
  ListNode *tmp = new ListNode(val);
  ListNode *cur = _head;
  for(int i = 0; i < _size; ++i)
  {
  cur = cur->next;
  }
  cur->next = tmp;
  ++_size;
  }

  void addAtIndex(int index, int val)
  {
  if(index > _size) return;
  if(index < 0) index = 0;
  ListNode *tmp = new ListNode(val);
  ListNode *p = _head;
  for(int i = 0; i < index; ++i)
  {
  p = p->next;
  }
  tmp->next = p->next;
  p->next = tmp;
  ++_size;
  }

  void deleteAtIndex(int index)
  {
  if (index >= _size || index < 0) {
  return;
  }
  ListNode *p = _head;
  for(int i = 0; i < index; ++i)
  {
  p = p->next;
  }
  ListNode *tmp = p->next;
  p->next = tmp->next;
  delete tmp;
  tmp = nullptr;
  --_size;
  }

  void printLinkedList() {
  ListNode* cur = _head;
  while (cur->next != nullptr) {
  cout << cur->next->val << " ";
  cur = cur->next;
  }
  cout << endl;
  }

private:
int _size;
ListNode *_head;
};

反转链表
使用头插法
/**

• Definition for singly-linked list.
• struct ListNode {

• int val;
  

• ListNode *next;
  

• ListNode() : val(0), next(nullptr) {}
  

• ListNode(int x) : val(x), next(nullptr) {}
  

• ListNode(int x, ListNode *next) : val(x), next(next) {}
  

• };
  /
  class Solution {
  public:
  ListNode reverseList(ListNode* head) {
  ListNode *p = head;
  ListNode *q = nullptr;
  if(head)
  {
  q = head->next;
  }
  while(q)
  {
  ListNode *tmp = q->next;
  q->next = head;
  p->next = tmp;
  head = q;
  q = p->next;
  }
  return head;
  }
  };