英语时差：如何找出最短路径（上）

If you ever had a paper route or any other job that took you through a series of streets, you may have spent some time working out the quickest way to complete the circuit. In order to cover your route most efficiently, you would want to avoid going back over streets you had already covered and you would want to avoid traveling on streets that were not part of your route. Children play a game in which one child draws a simple, geometric design on a piece of paper. The other then tries to trace the design with a pencil. The rules are that you can’t take the pencil off the paper and you can’t go over the same line twice. The catch is that not all designs can be traced this way. As you worked out the most efficient paper route, you might have tried this same exercise on a street map of the neighborhood you had to cover.