Buy Low Sell High: cf - 867E

Buy Low Sell High: cf - 867E
Let's maintain two multisets, "unused" and "already sold". After taking input "x", I tried to sell this pairing with a previous element(which I'm gonna buy). Took the minimum valued element among "unused" + "already sold" stuffs, let's call it "y".

If(y < x) {
 if(y came from unused set) buy y and sell x.
 else if(y came from "already sold" set) put y in unused set and sell x.
}
else put "x" in unused set
The solution actually comes from the idea — If I see that there's an "already sold" stuff smaller than x, it'd be optimal to sell "x" and not use y, rather than use y and buy x.