Best Time to Buy and Sell Stock using Strategy

Best Time to Buy and Sell Stock using Strategy

What is this problem about?

The "Best Time to Buy and Sell Stock using Strategy interview question" is a variant that introduces additional constraints or specific trading rules. While the core goal remains maximizing profit from stock prices, you might be required to follow a specific "strategy"—such as a limit on the number of days you can hold a stock, or a requirement to wait a certain number of days between trades.

Why is this asked in interviews?

Companies like Uber and Meta use the "Best Time to Buy and Sell Stock using Strategy coding problem" to see if a candidate can adapt a well-known algorithm to new rules. It tests flexibility and the ability to modify a "Sliding Window" or "Prefix Sum" approach. It evaluates whether you can think beyond the "standard" solutions and handle custom business logic.

Algorithmic pattern used

This variation often uses the Sliding Window or Prefix Sum pattern depending on the specific strategy.

• Fixed Holding Period: If you must sell exactly $D$ days after buying, you use a sliding window of size $D$.
• Minimum Profit Target: If you can only sell if the profit is at least $P$, you use a prefix minimum to find the first day that satisfies the condition.
• Prefix Sum: If the strategy involves cumulative gains or multiple segments, prefix sums can help calculate range totals in $O(1)$ time.

Example explanation

Strategy: "You must hold the stock for exactly 2 days." Prices: [1, 5, 2, 8, 3]

• Buy Day 0 (1), Sell Day 2 (2): Profit = 1.
• Buy Day 1 (5), Sell Day 3 (8): Profit = 3.
• Buy Day 2 (2), Sell Day 4 (3): Profit = 1. Max Profit using this strategy is 3.

Common mistakes candidates make

• Reverting to Stock I: Failing to apply the specific "strategy" rules and just solving the basic problem instead.
• Index Out of Bounds: Forgetting to handle the end of the array when a strategy requires looking ahead a certain number of days.
• Inefficiency: Using a nested loop when a sliding window or single pass would suffice.

Interview preparation tip

When an interviewer adds a "strategy" or a "twist" to a common problem, pay close attention to the constraints. Ask clarifying questions about the edge cases of the strategy. This shows you are thinking about the practical implementation, not just memorizing "Dynamic Programming" templates.