Swap Nodes in Pairs

Statement

Given a singly linked list, swap every two adjacent nodes of the linked list. After the swap, return the head of the linked list.

Note: Solve the problem without modifying the values in the list’s nodes. In other words, only the nodes themselves can be changed.

Constraints:

• The number of nodes in the list is in the range $[0, 100]$.
• $0 \leq$ Node.value $\leq 100$

Examples