If a graph can be drawn in one stroke, then for each vertex, the number of edges "entering" this point in the path is equal to the number of edges "leaving" this point: at this time, the degree of this point is even. Or the difference between the two is one: at this time, this point must be one of the starting point or the end point. Note that there must be an end point if there is a starting point, so the number of odd vertices is either 0 or 2.