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.
	 * 
	 * */