(Java) LeetCode 24. Swap Nodes in Pairs —— 两两交换链表中的节点

Given a linked list, swap every two adjacent nodes and return its head.

Example:

Given 1->2->3->4, you should return the list as 2->1->4->3.

Note:

• Your algorithm should use only constant extra space.
• You may not modify the values in the list's nodes, only nodes itself may be changed.

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本题是两个一组翻转链表，类似LeetCode 206. Reverse Linked List —— 反转链表。