/kloc-in the 0/8th century, there was a beautiful town in Europe-Konigsberg, where there were seven bridges. As shown in figure 1, there are two bridges connecting the small island A in the river with the left bank B and the right bank C of the river, and another bridge connecting the land D between the two tributaries of the river with A, B and C respectively. At that time, there was a question among the residents of Konigsberg: How can a person walk over seven bridges at a time, each bridge only once, and finally return to the starting point? Everyone is trying to find the answer to this question. But no one can solve it. .............................................................................................................................................................. Euler soon proved with profound insight that this way of walking does not exist. Euler solved the problem in this way: since land is the joint of bridges, we might as well regard the land separated by rivers as points A, B, C and D4, and express the seven bridges as seven lines connecting these four points. So the "Seven Bridges Problem" is equivalent to a problem of a graph. Euler noticed that if every point has an edge, then every point must have an edge, so every point must have an even number of edges to complete a stroke. Every point is connected by odd edges, so it is impossible to draw it with one stroke, that is to say, there is no way to cross seven bridges at a time, and each bridge can only cross once. Thus, the seven-bridge problem is solved and the geometric topology is derived.

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