unreasonably effective

link We can summarize all transactions as balances per person. In the end, there might be positive balances and negative balances, but they must sum up to 0. Blind alleys What works Try matching each positive balance with each negative balance making sure a transaction does not change a positive balance to negative or a…

link We can generate hours and minutes separately and take their Cartesian product to create valid times. For a given number of turned on hour (or minute) bits, we generate subsets of hour (or minute) bits. Time: , space: .

link The problem restricts placing one stone per move, otherwise it would have been harder. The problem here is to find the minimum total cost of assigning every empty cell to one extra stone. The cost of one such assignment is the Manhattan distance between the empty cell and the stone’s cell. We construct a…

link If source color and target color are the same, nothing to do. Otherwise, we remember the source color, color the cell and its four neighbors. A cell is filled at most once. Time: , space: .