day4打卡

两两交换链表中的节点
一开始有错误，找不出来，但是gdb真好用

• 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 swapPairs(ListNode* head) {
  ListNode *dummyhead = new ListNode(0);
  dummyhead->next = head;
  ListNode *cur = dummyhead;
  while(cur->next && cur->next->next)
  {
  ListNode tmp1 = cur->next->next->next;
  ListNode tmp2 = cur->next;
  cur->next = cur->next->next;
  cur->next->next = tmp2;
  cur->next->next->next = tmp1;
  cur = cur->next->next;
  }
  return dummyhead->next;
  }
  };
  删除倒数第N个节点
  一开始没有考虑到删除最后一个节点
  /

• 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 removeNthFromEnd(ListNode* head, int n) {
  ListNode *dummyhead = new ListNode(0);
  dummyhead->next = head;
  ListNode *p = dummyhead;
  ListNode q = p;
  for(int i = 0; i < n; ++i)
  {
  q = q->next;
  }
  while(q->next)
  {
  p = p->next;
  q = q->next;
  }
  ListNode tmp = p->next;
  if(n == 1)
  {
  p->next =nullptr;
  }
  else
  {
  p->next = p->next->next;
  }
  delete tmp;
  return dummyhead->next;
  }
  };
  链表相交
  /

• Definition for singly-linked list.

• struct ListNode {

• int val;
  

• ListNode *next;
  

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

• };
  */
  class Solution {
  public:
  ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
  if(headA == NULL || headB == NULL)
  {
  return NULL;
  }
  ListNode *cur = headA;
  int len1 = 0;
  int len2 = 0;
  while(cur)
  {
  len1++;
  cur = cur->next;
  }
  cur = headB;
  while(cur)
  {
  len2++;
  cur = cur->next;
  }
  ListNode *p = headA;
  ListNode *q = headB;
  if(len1 > len2)
  {
  int len = len1 -len2;
  for(int i = 0; i < len; ++i)
  {
  p = p->next;
  }
  }
  else
  {
  int len = len2 - len1;
  for(int i = 0; i < len; ++i)
  {
  q = q->next;
  }
  }
  while(p && p != q)
  {
  p = p->next;
  q = q->next;
  }
  return p;

  }
  };

环形链表
/**

• Definition for singly-linked list.

• struct ListNode {

• int val;
  

• ListNode *next;
  

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

• };
  */
  class Solution {
  public:
  ListNode *detectCycle(ListNode *head) {
  ListNode *p = head;
  ListNode *q = head;

   while(q && q->next)
   {
       p = p->next;
       q = q->next->next;
       if(q == p)
       {
           ListNode *p1 = head;
           ListNode *q1 = q;
           while(p1 != q1)
           {
               p1 = p1->next;
               q1 = q1->next;
           }
           return p1;
       }
   }
   return nullptr;
  

}
};