Given a linked list, swap every two adjacent nodes and return its head.
For example,
Given 1->2->3->4
, you should return the list as 2->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.
1 /** 2 * Definition for singly-linked list. 3 * struct ListNode { 4 * int val; 5 * ListNode *next; 6 * ListNode(int x) : val(x), next(NULL) {} 7 * }; 8 */ 9 class Solution { 10 public: 11 ListNode* swapPairs(ListNode* head) { 12 if(head==NULL || head->next==NULL) return head; 13 ListNode *pre=head; 14 ListNode *cur=head; 15 ListNode *tmp; 16 while(cur && cur->next) 17 { 18 tmp = cur->next; 19 cur->next = cur->next->next; 20 tmp->next = cur; 21 if(pre==head) 22 head = tmp; 23 else 24 pre->next = tmp; 25 pre=cur; 26 cur = cur->next; 27 } 28 return head; 29 } 30 };