对于给定的图G，如果存在一种用2种颜色对顶点着色的方案，使得图中任意一条边所连接的2 个顶点着有不同颜色，则称图G 是可2着色的。-For a given graph G, if there is a vertex with two colors on the color scheme, making the graph an edge is connected to any two vertices with different colors, called graph G is 2 coloring.