One Line — free browser game

A tiny cousin of the Königsberg Bridges problem: draw all the lines connecting the nodes on screen (the filled circles) without lifting your pen and without crossing any line twice. To start, click or tap any node — the node you pick is highlighted. Then tap one of the neighbouring nodes connected to it by a straight line: the line between the two is painted in the highlight colour and your pen has slid over there. On the next step you can only move to the as-yet-unused neighbours of the node where your pen is resting; jumping to nodes that aren't diagonally adjacent or directly connected is not allowed. If you tap a wrong node the move is rejected and the level state is not disturbed. 'Undo' (the U key or button) gives back the last line you drew; 'Reset' (R) starts the level over from the beginning. When all the lines are painted you have completed the level: your score goes up by +1, your highest level number is recorded, and with 'Next' (N) you move on to a graph that is one step more complex. In some graphs a solution is only possible by starting from certain nodes (Euler's theorem: you must start from the odd-degree nodes). If you feel stuck, undo or reset and try a different starting point. Tip: first count by eye the number of lines connected to each node — if exactly two nodes have an odd connection count and all the rest are even, start from one of those two nodes; if all counts are even, start wherever you like. If there are three odd-degree nodes the graph is unsolvable, but in this puzzle you are always given one that can be solved.