Create Swap_Nodes_in_Pairs

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.(自己也要测试全了，测测长一点的，才清楚指针是不是对的。)