Given the head of a singly linked list, swap every two adjacent nodes and return the new head. Nodes must actually be relinked, not just have their values swapped — a leftover single node (odd-length list) stays exactly where it is. The optimal solution anchors a prev pointer right before each pair with a dummy node, exactly the same idea used to reverse any run of nodes: relink first.next, second.next, and prev.next in a fixed three-line sequence so second ends up leading and first trails right behind it, then slide prev up to first — now the trailing node of the pair just swapped — before repeating for the next pair.