Leetcode ---- Swap Nodes in Pairs

题目链接

题意：给出单链表头指针，要求交换链表中每一对相邻的结点．

注意：不可以改变链表中结点的值，只可以使用常量空间．

 

附上代码：

 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) { 13 return head; 14  } 15 ListNode *p = head, *q = head->next, *pre = head; 16 while (q != NULL) { 17 p->next = q->next; 18 q->next = p; 19 if (pre == head) { 20 head = q; 21 } else { 22 pre->next = q; 23  } 24  swap(q, p); 25 pre = q; 26 p = q->next; 27 if (p == NULL) { 28 break; 29  } 30 q = q->next->next; 31  } 32 33 return head; 34  } 35 };