Euler method to analyze this problem is to Konigsberg Town A, B, C, D regarded as four vertices, each bridge connecting the city's edge are treated, so the problem is reduced to: each vertex from the figure, whether there is a path through each edge exactly once, and then back to square one? If this path is present, then for each vertex is concerned, there is an incoming vertex edge, there is a side drawn from the apex, so each vertex must have an even number of sides, however, carefully observed: A , B, C, D is not an even number of four edge vertices, and therefore, this path does not exist. Seven Bridges problem is a classical problem in graph theory is an important theory in graph theory - the theory of simply connected graph.