Well Well Well
A Jane Street puzzle: a 7×7 well with per-cell depths 1 through 49 is filled at 1 cubic foot per minute from the cell marked 1; water disperses evenly among adjacent lower-depth regions. After how many minutes does the water level in cell 43 start to rise?
NetworkX + exact rational arithmetic (Fraction) models the water distribution. Water entering a flat region disperses among its deepest neighbours equally; when a region has no deeper neighbour, it fills until its depth matches the next. Simulation yields 360 minutes. Full code and the final depth grid are in the PDF.