Water Bottles II is an iteration on the original simulation problem with a twist: the exchange rate is not constant. Every time you exchange empty bottles for a full one, the number of empty bottles required for the next exchange increases by one. This added complexity requires more careful step-by-step tracking of the exchange threshold.
Meta and Bloomberg use this to see if a candidate can handle evolving constraints. It's a test of state management. While the first version could potentially be solved with a mathematical formula, this version almost always requires a Simulation approach because the "price" of a bottle changes over time. It evaluates your ability to implement logic that changes based on previous actions.
This is a dynamic Simulation interview pattern. Unlike the static version, you cannot use a single division to find the result. You must use a loop where, in each step, you check if your empty_bottles >= current_exchange_rate. If it is, you subtract the rate from your empty bottles, add one to your full bottles, and then increment the current_exchange_rate.
Initial: 10 full bottles, exchange rate starts at 3.
The most common mistake is forgetting to increment the exchange rate after each swap. Another pitfall is drinking all bottles first and then trying to do all exchanges at once, which doesn't work here because the rate changes per bottle exchanged, not per batch.
Always trace your logic with a small example when the rules change dynamically. This helps you catch off-by-one errors or forgotten increments. In a real interview, talking through the state changes at each step shows the interviewer you have a firm grasp of the problem's mechanics.
