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.
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{% codesnippet "./code/swap-nodes-in-pairs-1."+book.suffix, language=book.suffix %}{% endcodesnippet %}
下面这种写法更简洁，但题目规定了不准这样做。
{% codesnippet "./code/swap-nodes-in-pairs-2."+book.suffix, language=book.suffix %}{% endcodesnippet %}