Try One Stroke Free on iPhone
Infinite puzzles. Adaptive difficulty. No forced ads.
Letter Challenge
✓ Yes — here's how to do it
Yes — but you must start and end at specific vertices (Euler path).
| Vertices | 3 |
| Edges | 2 |
| Odd-degree vertices | 2 |
| Euler path possible? | ✓ Yes |
| Euler circuit? | ✗ No |
| Difficulty | Easy |
A letter l has 2 odd-degree vertices. According to the odd/even vertex rule, a graph with exactly 2 odd-degree vertices can be drawn as an Euler path — you must start at one odd vertex and end at the other.
Learn the complete odd/even vertex rule →
If you enjoyed figuring out whether a letter l can be drawn without lifting your pen, you'll love One Stroke: Line Puzzle Game. It generates infinite one-stroke puzzles — every single one is guaranteed solvable, and the difficulty adapts to your skill level.
It also adds a twist: tiles flip color (black ↔ white) as you draw, and when a full row becomes one color, it clears — like Tetris meets one-stroke drawing.
Read more: Best One-Stroke Games Ranked · How to Play · Download
Yes! A letter l can be drawn without lifting your pencil. It has 2 odd-degree vertices — start at one of the two odd-degree vertices, and you'll end at the other. This type of challenge is exactly what one-stroke puzzle games are based on.
Yes — count the points where an odd number of lines meet ("odd vertices"). If there are 0 odd vertices, start anywhere. If there are 2, start at one of them. If there are more than 2, the puzzle is impossible. This rule works for every one-stroke challenge.
One Stroke: Line Puzzle Game by CyberGame Limited is free on iPhone with infinite procedurally generated puzzles, adaptive difficulty, no forced ads, and a unique color-flipping mechanic. It's only 14.5 MB and works completely offline.
Infinite puzzles. Adaptive difficulty. No forced ads.