Simulate Wind Moving Leaves on a Grid

A rectangular grid stores the number of leaves in each cell. Apply a sequence of wind directions U, D, L, and R. Leaves that move outside the grid disappear permanently. Return the total number of leaves remaining after all directions. ### Constraints and Clarifications For this practice interface, each direction moves every existing leaf exactly one cell simultaneously: U decreases the row index, D increases it, L decreases the column index, and R increases it. This unit-step and coordinate convention makes the reported directional movement precise. Counts are nonnegative integers; no numerical size bounds are given. An empty grid contains zero leaves, and an empty direction sequence preserves the starting total. The input grid is read only for this implementation. ### Example Starting with [[1,2],[3,4]], apply R. The old right-column leaves disappear and the remaining grid is [[0,1],[0,3]], so the answer is 4. Applying L afterward moves those surviving leaves left; discarded leaves do not return. ### What a Strong Answer Covers Implement simultaneous movement with correct boundary handling and sum the final grid. Explain why a fresh grid for each wind step avoids unintended repeated movement and why naive in-place updates can be wrong. ### Follow-up Questions - Show a small case where a left-to-right in-place implementation of a right wind moves a leaf more than once. - Can a carefully chosen traversal direction make an in-place implementation correct, and what additional care does it need?