Codeforces 865D Buy Low Sell High - The Idea

Use a min-heap to track all previous prices. When you see price $p_i$, you have two choices: buy at $p_i$ or sell shares bought earlier.

If $p_i > \text{heap.top()}$, you can profit by selling. Pop the smallest price from the heap (the buy price), add $p_i - \text{heap.top()}$ to profit. Then push $p_i$ twice: once as the sell price just used, once as a potential buy for the future. This "undoes" the sell if a better opportunity comes.