grid holds 0 for an empty cell and 1 for an obstacle. You start at the top-left cell and want to reach the bottom-right cell, moving up, down, left or right. You may walk through at most k obstacles, removing each one as you step onto it.
Return the fewest steps to reach the bottom-right cell, or -1 if it cannot be reached. Both corner cells are empty.
- Arriving at a cell with more removals left is better than arriving with fewer, so a visited mark on the cell alone loses answers
- A state is the cell together with the number of removals left