Outside Sudoku - 200 Hard to Master Puzzles 9x9 (Volume 7)

Sudoku (also known as "Number Place") is a logic-based, combinatorial number-placement puzzle. The objective is to fill a 9x9 grid with digits so that each column, each row, and each of the nine 3x3 subgrids that compose the grid (also called "boxes", "blocks", or "regions") contains all of the digits from 1 to 9. The puzzle setter provides a partially completed grid, which for a well-posed puzzle has a single solution. Outside Sudoku is a variation of the standard sudoku. Some digits are given outside the grid. These digits must appear in the first three cells in the corresponding direction.