Graph theory originated in18th century. 1736, the Swiss mathematician Euler published the first graph theory paper "Seven Bridges in Konigsberg". In the city of Konigsberg at that time, there was a Fritz fritz pregl River that ran across the city. The two islands in the river are connected with the banks by seven bridges. At that time, the residents there were keen on a difficult problem: how some tourists walked around the Seven Bridges without repeating them, and finally returned to the starting point. In order to solve this problem, Euler used letters A, B, C and D4 instead of land as four vertices, and used corresponding line segments to represent the bridge connecting the two lands, so the problem of the Seven Bridges in Konigsberg became whether there is a loop, which passes through each edge once and only once, and all vertices. Euler pointed out in his paper that such a ring does not exist.