Alternating cycle in a graph Consider a connected graph of $n\ge 7$ vertices. The edges are colored in two colors, and for each vertex, there is equal number of edges of each color connected to it. There is also a vertex with 6 edges connected to   it. 
Is it true that in this graph, there exists a cycle that alternates between the two colors? 
 A: The properties of the graph assure that we can start from a vertex $v$ of degree six and go along the edges, in each vertex choosing the next edge to go which was not passed before and is colored differently to the color of the incoming edge, until we reach the vertex $v$ again. Denote the cycle (not necessarily simple) which we just passed as $C_1$ and remove all its edges from the graph. Again start from the vertex $v$ and go along the remaining edges according to the previous rules, until we reach the vertex $v$ again. Denote the cycle (not necessarily simple) which we just passed as $C_2$. Repeat the above process again, obtaining a cycle $C_3$. The construction of the cycles $C_i$ assures that each vertex of $C_i$ different from $v$ is incident to differently colored edges of $C_i$. If in each of the cycles $C_i$ the vertex $v$ is incident to similarly colored edges of $C_i$ then the vertex $v$ is incident in total to an even number of edges of each color, which is impossible, because this number is $3$. Thus there exists a cycle $C_i$ each vertex of which is incident to differently colored edges of $C_i$.
The following example shows that there may be no alternating cycle visiting each its vertex only once.

