check if a part, there exists a cell which has only one valid number.

checks if two cells in same row or same column, have same number in the valid, and that number does not exist as part of valid in same cell.

returns the part number this cell belongs too panic if the cell is out of bounds.

returns the top,left cell for the given part panics for out of bounds part.

assuming meta for grid is already updated.

grid is divided into 9 parts, each part of 9 cells each [0, 1, 2] [3, 4, 5] [6, 7, 8].