Plant Infection (multi-part)

You are given a plant field where some plants are initially infected. Infection spreads to neighboring cells according to a fixed rule (typically level-by-level / simultaneous spread). Based on the same base setting, the interviewer asks 5 progressively harder sub-questions (e.g., time to infect all plants or return a sentinel if impossible; earliest infection time for each cell; multi-source infection; obstacles that block spread; queries about infection status by day/position, etc.). Note: The post does not provide the exact 5 sub-questions nor the precise input/output format. The above is a faithful high-level summary of the common interview pattern; with more details, it can be turned into a fully specified problem statement.