“A robot is located at the top-left corner of a 5×5 grid. The robot can move either up, down, left, or right, but can not visit the same spot twice. The robot is trying to reach the bottom-right corner of the grid.” (link)
How many unique paths can the robot walk?
One simple solution is to recursively walk the grid and eliminate as much one-ways as possible.
Hint: Time measurement on Core2Quad Q9400, 8GB RAM