Implement BFS and a Job Scheduler

The interview report described two coding questions in the same category:

1. Grid reveal with BFS You are given an m x n board representing a hidden minefield. Each cell is one of:
   • M : unrevealed mine
   • E : unrevealed empty cell
   • B : revealed blank cell with no adjacent mines
   • 1 - 8 : revealed cell showing the number of adjacent mines
   • X : revealed mine after a click
   Given a click position (row, col) , update the board using these rules:
   • If the clicked cell is a mine ( M ), change it to X .
   • Otherwise, reveal cells using BFS :
     • Count the number of mines in the 8 neighboring cells.
     • If the count is greater than 0, write that digit in the cell and stop expanding from it.
     • If the count is 0, mark the cell as B and continue revealing its unrevealed neighbors.
   Return the updated board.
2. Concurrent periodic job scheduler Implement a thread-safe in-memory scheduler that supports periodic jobs. Each job has:
   • a unique jobId
   • an execution interval such as every N milliseconds
   • a callback function to run
   Support the following operations:
   • register(jobId, intervalMs, task)
   • cancel(jobId)
   • start()
   • stop()
   Requirements:
   • Multiple worker threads may execute jobs concurrently.
   • A job should be triggered approximately once per interval.
   • The same scheduled run must not execute more than once.
   • Cancellation must be safe even if other threads are polling or executing jobs.
   • Explain how you would handle cases where a task runs longer than its interval, clock drift, and backpressure under heavy load.