Swap Nodes in Pairs

Problem

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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Idea

Be careful with the pointer.

The number of nodes in the list may be odd.

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Solution

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