leetCode_swap_pairs

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background

LeetCode24: swap pairs

intuitive sol

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  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) {}
  };
#include <iostream>
class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        ListNode *dummy = new ListNode(-1);
        ListNode *prev = dummy ;
        prev->next = head ;
        ListNode *p1=head, *p2=p1->next, *p3=p2->next, *p4=p3->next, *pn, *tmp ;
        if(p1 == nullptr)
        {
            return nullptr ;
        }else if(p2 == nullptr){
            return p1 ;
        }else if(p3 == nullptr)
        {
            p2->next = p1 ;
            p1->next = nullptr ;
            head = p2 ;
            return head ;
        }else if(p4 == nullptr)
        {
            head->next = p2 ;
            p2->next = p1 ;
            p1 ->next = p3;
            return head ;
        }
        for(p1=head, p2=p1->next, p3=p2->next, p4=p3->next ; p4 && p3 && p2 && p1;  prev=p3, p1=prev->next, p2=p1->next, p3=p2->next, p4=p3->next )
{
   pn = p4->next ;
   p2->next = p1 ;
   p4->next = p3 ;
   p1->next = p4;
   p3->next = pn ;
   prev->next = p2 ;
   if (pn == nullptr)
   {
     break;
   }else if(pn->next == nullptr)
   {
     break;
   }else if (pn->next->next == nullptr)
   {
     break;
   }else if(pn->next->next->next == nullptr)
   {
     break;
   }
}
  if(pn == nullptr || pn->next == nullptr){
    return prev->next;
  }
   if(pn->next->next == nullptr )
   {
     tmp = pn->next ;
     prev->next = pn->next ;
     tmp->next = pn ;
     pn->next = nullptr;
   }else if(pn->next->next->next == nullptr){
     tmp = pn->next ;
     ListNode *last = tmp->next ;
     prev->next = tmp ;
     pn->next = last ;
   }
     return prev->next;
    }
};
int main()
{
   ListNode *head = new ListNode(1);
   ListNode *sec = new ListNode(2);
   ListNode *thd = new ListNode(3);
   ListNode *fth = new ListNode(4);
   head->next = sec ;
   sec->next = thd ;
   thd->next = fth ;
   Solution *sol = new Solution();
   ListNode *res = sol->swapPairs(head) ;
   std::cout << res->val << ', ' << res->next->val << ', ' << res->next->next->val << std::endl;
   return 0;
}