24. Swap Nodes in Pairs

Given a linked list, swap every two adjacent nodes and return its head.
For example,Given1->2->3->4
, you should return the list as2->1->4->3
Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.

class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        if(head==NULL||head->next==NULL)
           return head;
        ListNode* fakehead = new ListNode(-1);
        fakehead->next = head; //牢记这种方法
        
        ListNode* ptr1 = fakehead;
        ListNode* ptr2 = head;
        
        while(ptr2!=NULL&&ptr2->next!=NULL)
        {
            ListNode* nextstart = ptr2->next->next; //自己画图，设定各个指针，然后找出它们之间的关系，关键是链表不能断
            ListNode* pnext = ptr2->next;  //自己画图，设定各个指针，然后找出它们之间的关系，关键是链表不能断
            ptr2->next = nextstart;
            pnext->next = ptr2;
            ptr1->next = pnext;
            
            ptr1 = ptr2;
            ptr2 = nextstart;
            
        }
        
        return fakehead->next;
    }
};