Best Time to Buy and Sell Stock with Cooldown

Write `max_profit_cooldown(prices)` returning the maximum profit with unlimited transactions, except that you may not buy on the day after you sell. max_profit_cooldown([1,2,3,0,2]) -> 3 (buy 1, sell 2, cooldown, buy 0, sell 2) Model it as three states — holding, just sold, and free to buy — and write the transition into each. Once the states are named the code is mechanical; trying to reason about it without naming them is where people get lost.