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.

Solution:

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode swapPairs(ListNode head) {
        ListNode p=new ListNode(0),s;
        p.next=head;
        head=p;
        while(p.next!=null && p.next.next!=null){
            s=p.next.next;
            p.next.next=s.next;
            s.next=p.next;
            p.next=s;
            p=s.next;
        }
        return head.next;
    }
}

C语言源代码（1ms）不开辟新的节点内存：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* swapPairs(struct ListNode* head) {
    struct ListNode *p=head,*s;
    if(p!=NULL && p->next!=NULL){
        s=p->next;
        p->next=s->next;
        s->next=p;
        head=s;
        while(p->next!=NULL && p->next->next!=NULL){
            s=p->next->next;
            p->next->next=s->next;
            s->next=p->next;
            p->next=s;
            p=s->next;
        }
    }
    return head;
}

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        ListNode *p,*s;
        p=new ListNode(0);
        p->next=head;
        head=p;
        while(p->next!=NULL && p->next->next!=NULL){
            s=p->next->next;
            p->next->next=s->next;
            s->next=p->next;
            p->next=s;
            p=s->next;
        }
        return head->next;
    }
};

Python源代码（81ms）：

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    # @param {ListNode} head
    # @return {ListNode}
    def swapPairs(self, head):
        p=ListNode(0)
        p.next=head;head=p
        while p.next!=None and p.next.next!=None:
            s=p.next.next
            p.next.next=s.next
            s.next=p.next
            p.next=s
            p=s.next
        return head.next