Imagine that you invest in the stock market. The value of each stock can vary daily, and it might differ from its original purchase cost. The net worth of your stock portfolio is the difference between its original cost and its current value. Design a system to track your investments. Include algorithms that purchase stock, sell stock from your portfolio, and calculate the current net worth of the portfolio. When you sell a stock, you sell the most recently purchased one first.
Hint: Each purchase you make consists of n shares of stock, each at a cost of d dollars. You can place each purchase on a stack. If only a portion of a particular purchase is sold, remove the purchase entry from the stack, modify it to reflect the remaining shares, and place it back onto the stack.