Simulate Water Flow

Problem Overview

• Simulate rainfall on a 2D elevation grid where water moves to adjacent cells of equal or lower height.
• From startRow, startCol, return a grid of the first time step each cell gets wet (0 at start, -1 if never).
• Input: integer matrix and start coordinates; Output: same-size timestamp matrix; scalable to large grids.
• Real-world domain: civil engineering and digital elevation models.
• From Uber interviews: a coding interview problem and common interview question.