Single-Threaded CPU

You are given `n`​​​​​​ tasks labeled from `0` to `n - 1` represented by a 2D integer array `tasks`, where `tasks[i] = [enqueueTimei, processingTimei]` means that the `i​​​​​​th`​​​​ task will be available to process at `enqueueTimei` and will take `processingTimei` to finish processing. You have a single-threaded CPU that can process **at most one** task at a time and will act in the following way: - If the CPU is idle and there are no available tasks to process, the CPU remains idle. - If the CPU is idle and there are available tasks, the CPU will choose the one with the **shortest processing time**. If multiple tasks have the same shortest processing time, it will choose the task with the smallest index. - Once a task is started, the CPU will **process the entire task** without stopping. - The CPU can finish a task then start a new one instantly. Return _the order in which the CPU will process the tasks._   **Example 1:** **Input:** tasks = [[1,2],[2,4],[3,2],[4,1]] **Output:** [0,2,3,1] **Explanation: **The events go as follows: - At time = 1, task 0 is available to process. Available tasks = {0}. - Also at time = 1, the idle CPU starts processing task 0. Available tasks = {}. - At time = 2, task 1 is available to process. Available tasks = {1}. - At time = 3, task 2 is available to process. Available tasks = {1, 2}. - Also at time = 3, the CPU finishes task 0 and starts processing task 2 as it is the shortest. Available tasks = {1}. - At time = 4, task 3 is available to process. Available tasks = {1, 3}. - At time = 5, the CPU finishes task 2 and starts processing task 3 as it is the shortest. Available tasks = {1}. - At time = 6, the CPU finishes task 3 and starts processing task 1. Available tasks = {}. - At time = 10, the CPU finishes task 1 and becomes idle. ``` **Example 2:** **Input:** tasks = [[7,10],[7,12],[7,5],[7,4],[7,2]] **Output:** [4,3,2,0,1] **Explanation****: **The events go as follows: - At time = 7, all the tasks become available. Available tasks = {0,1,2,3,4}. - Also at time = 7, the idle CPU starts processing task 4. Available tasks = {0,1,2,3}. - At time = 9, the CPU finishes task 4 and starts processing task 3. Available tasks = {0,1,2}. - At time = 13, the CPU finishes task 3 and starts processing task 2. Available tasks = {0,1}. - At time = 18, the CPU finishes task 2 and starts processing task 0. Available tasks = {1}. - At time = 28, the CPU finishes task 0 and starts processing task 1. Available tasks = {}. - At time = 40, the CPU finishes task 1 and becomes idle. ```   **Constraints:** - `tasks.length == n` - `1 <= n <= 105` - `1 <= enqueueTimei, processingTimei <= 109`