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 the given graph G, if there is a use two colors of vertex programs, make a map arbitrary edge connected with two vertices are different colors, says G is 2-colored.