980. Unique Paths III

You are given an m x n integer array grid where grid[i][j] could be: - 1 representing the starting square. There is exactly one starting square. - 2 representing the ending square. There is exactly one ending square. - 0 representing empty squares we can walk over. - -1 representing obstacles that we cannot walk over. Return the number of 4-directional walks from the starting square to the en