Replay a log that can take it back
A stock adjustment screen records every change, and an undo command that cancels whichever change came last.
- A command is either a signed whole number to apply, or the word "undo".
- An undo cancels the most recent change that is still standing.
- An undo with nothing left to cancel does nothing.
- Start from zero and return the total once every command has run.
replayLog(commands: list<string>) → int
Java needs a compiler and Drill does not host one yet, so this page is the reference rather than an exercise: the problem, worked examples, and the solution in full. To type it out, the same problem runs in Python.
Where you start
int replayLog(List<String> commands) {
}Worked examples
| Call | Result |
|---|---|
replayLog(Main.<String>ls("5", "3", "undo")) | 5 |
replayLog(Main.<String>ls("5", "undo", "undo")) | 0 |
replayLog(Main.<String>ls("10", "-4", "2")) | 8 |
replayLog(Main.<String>ls()) | 0 |
Hint
Keep the applied changes on a stack. Undo pops the last one off and subtracts it back out.
Reference solution in Java
int replayLog(List<String> commands) {
Deque<Integer> applied = new ArrayDeque<>();
int total = 0;
for (String command : commands) {
if (command.equals("undo")) {
if (!applied.isEmpty()) total -= applied.pop();
} else {
int amount = Integer.parseInt(command);
applied.push(amount);
total += amount;
}
}
return total;
}