21.两两交换链表中的节点

class Solution {
    public ListNode swapPairs(ListNode head) {
        ListNode a=head,b=head;
        while(a!=null&&a.next!=null){
            b=a.next;
            //两两交换
            int temp;
            temp=a.val;
            a.val=b.val;
            b.val=temp;
            //向后移位
            a=b.next;
        }
        return head;
    }
}

class Solution(object):
    def swapPairs(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        a=head
        b=head
        while(a and a.next):
            b=a.next
            a.val,b.val=b.val,a.val
            a=b.next
        return head