给定一个链表，交换相邻2个节点的值。
举例：
输入:
1->2->3->4
输出:
2->1->4->3
class Node:
def init(self, data, next):
self.data = data
self.next = next
def swap_nodes_in_two(root):
prev = root
while prev and prev.next:
next = prev.next
tmp = prev.data
prev.data = next.data
next.data = tmp
prev = next.next
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