The Konigsberg bridges problem, something of an 18th-century oddity, was solved by the Swiss mathematician Leonhard Euler in 1736. It is an early example of the way Euler used ideas of what we now call graph theory to solve topological problems.
The River Pregel running through Konigsberg (now in the Soviet Union and called Kaliningrad) contained two islands. The islands and the two pieces of mainland were connected by seven bridges. The good burghers of the town, on their Sunday stroll, used to try to cross all seven bridges once without crossing any bridge twice. The problem was to find such a route.
Euler simplified the problem to a network of points, representing landmasses, connected by lines, representing bridges. He proved the hoped-for journey was impossible because every point, or landmass, had an odd number of lines, or bridges, connected to it and would therefore have to be either the start or finish of the walk.
