24. Swap Nodes in Pairs

 1 class Solution {
 2 public:
 3 ListNode* swapPairs(ListNode* head) {
 4 ListNode *t1,*t2,*t3,*t4;
 5 if(head==NULL||head->next==NULL)
 6     return head;                   
 7 t4->next=t2->next;
 8 t3=t2->next->next;
 9 t2->next->next=t2;
10 t2->next=t3;
11 while(t2->next->next!=NULL)   
12 {
13 t1=t2->next;
14 t3=t2->next->next->next;
15 t2->next=t2->next->next;
16 t2->next->next=t1;
17 t1->next=t3;
18 t2=t1;
19 }
20 } 
21 }