One should always begin the solution of a Sudoku puzzle by looking for cells within boxes to
enter numbers within that box that are missing.The easiest approach is to begin with the highest
frequency number(s) given in the puzzle. The method proceeds by finding a box that is missing this high-frequency number and determining whether there is one and only one cell into which
this number can be entered. If such a cell exists, the entry of that number into the cell is forced. When no more of this number can be forced, continue with the number with the next highest frequency
number and so forth until no more numbers can be forced into cells