Simulate a Stack from Push, Pop and End-of-Input Commands

Simulate a stack driven by a stream of text commands and report what it outputs. The commands arrive as one string of whitespace-separated tokens: - `U num`: push the integer `num` onto the stack. - `O`: pop the top value off the stack and output it. - `X`: the input stream has ended. Return the output values in the order they were popped. ### Function Signature ```python def simulate_stack(commands: str) -> list[int]: ``` ### Rules - Tokens are separated by one or more whitespace characters (spaces or newlines). - `U` is always followed by exactly one integer token, written in decimal with an optional leading `-`. - Each `O` removes the current top value and appends it to the output. - `X` appears exactly once, as the last token. Values still on the stack when `X` is reached are not output. - The original prompt printed the popped values separated by spaces, so the printed output `-3 3` corresponds to the return value `[-3, 3]`. ### Constraints - The input has between 1 and 200,000 tokens. - `-10^9 <= num <= 10^9` - Every `O` is issued while the stack is non-empty. ### Examples **Example 1** ```text Input: commands = "U 3 U -3 O O X" Output: [-3, 3] ``` `3` is pushed, then `-3`. The first `O` pops `-3` and the second pops `3`. **Example 2** ```text Input: commands = "U 5 U 7 O U 2 X" Output: [7] ``` Only one pop happens. `5` and `2` are still on the stack at `X`, so they are not output. **Example 3** ```text Input: commands = "X" Output: [] ``` The stream ends before any command runs.