The math winter homework of Mr. Han's grandson is for the first grade of elementary school under the Beijing Normal University version. The problem shown above is titled "Mailman Delivering Letters."
Referring to the figure in the upper right corner, the mailman has to deliver letters to various locations (the symbol "·" represents a delivery point). The roads are difficult to navigate (one must pass through a blank square "□" between two delivery points and cannot travel diagonally), and he also needs to go around buildings. Before setting out, he designed a route to deliver letters starting from the post office, ensuring that each location receives its letters without repeating any part of the route, and finally returning to the post office. Draw the mailman's walking route on the diagram.