diff --git a/backtracking/sudoku.py b/backtracking/sudoku.py new file mode 100644 index 000000000..b33351fd4 --- /dev/null +++ b/backtracking/sudoku.py @@ -0,0 +1,151 @@ +""" + + Given a partially filled 9×9 2D array, the objective is to fill a 9×9 + square grid with digits numbered 1 to 9, so that every row, column, and + and each of the nine 3×3 sub-grids contains all of the digits. + + This can be solved using Backtracking and is similar to n-queens. + We check to see if a cell is safe or not and recursively call the + function on the next column to see if it returns True. if yes, we + have solved the puzzle. else, we backtrack and place another number + in that cell and repeat this process. + +""" + +# assigning initial values to the grid +initial_grid = [ + [3, 0, 6, 5, 0, 8, 4, 0, 0], + [5, 2, 0, 0, 0, 0, 0, 0, 0], + [0, 8, 7, 0, 0, 0, 0, 3, 1], + [0, 0, 3, 0, 1, 0, 0, 8, 0], + [9, 0, 0, 8, 6, 3, 0, 0, 5], + [0, 5, 0, 0, 9, 0, 6, 0, 0], + [1, 3, 0, 0, 0, 0, 2, 5, 0], + [0, 0, 0, 0, 0, 0, 0, 7, 4], + [0, 0, 5, 2, 0, 6, 3, 0, 0], +] +# a grid with no solution +no_solution = [ + [5, 0, 6, 5, 0, 8, 4, 0, 3], + [5, 2, 0, 0, 0, 0, 0, 0, 2], + [1, 8, 7, 0, 0, 0, 0, 3, 1], + [0, 0, 3, 0, 1, 0, 0, 8, 0], + [9, 0, 0, 8, 6, 3, 0, 0, 5], + [0, 5, 0, 0, 9, 0, 6, 0, 0], + [1, 3, 0, 0, 0, 0, 2, 5, 0], + [0, 0, 0, 0, 0, 0, 0, 7, 4], + [0, 0, 5, 2, 0, 6, 3, 0, 0], +] + + +def is_safe(grid, row, column, n): + """ + This function checks the grid to see if each row, + column, and the 3x3 subgrids contain the digit 'n'. + It returns False if it is not 'safe' (a duplicate digit + is found) else returns True if it is 'safe' + + """ + + for i in range(9): + if grid[row][i] == n or grid[i][column] == n: + return False + + for i in range(3): + for j in range(3): + if grid[(row - row % 3) + i][(column - column % 3) + j] == n: + return False + + return True + + +def is_completed(grid): + """ + This function checks if the puzzle is completed or not. + it is completed when all the cells are assigned with a number(not zero) + and There is no repeating number in any column, row or 3x3 subgrid. + + """ + + for row in grid: + for cell in row: + if cell == 0: + return False + + return True + + +def find_empty_location(grid): + """ + This function finds an empty location so that we can assign a number + for that particular row and column. + + """ + + for i in range(9): + for j in range(9): + if grid[i][j] == 0: + return i, j + + +def sudoku(grid): + """ + Takes a partially filled-in grid and attempts to assign values to + all unassigned locations in such a way to meet the requirements + for Sudoku solution (non-duplication across rows, columns, and boxes) + + >>> sudoku(initial_grid) # doctest: +NORMALIZE_WHITESPACE + [[3, 1, 6, 5, 7, 8, 4, 9, 2], + [5, 2, 9, 1, 3, 4, 7, 6, 8], + [4, 8, 7, 6, 2, 9, 5, 3, 1], + [2, 6, 3, 4, 1, 5, 9, 8, 7], + [9, 7, 4, 8, 6, 3, 1, 2, 5], + [8, 5, 1, 7, 9, 2, 6, 4, 3], + [1, 3, 8, 9, 4, 7, 2, 5, 6], + [6, 9, 2, 3, 5, 1, 8, 7, 4], + [7, 4, 5, 2, 8, 6, 3, 1, 9]] + >>> sudoku(no_solution) + False + """ + + if is_completed(grid): + return grid + + row, column = find_empty_location(grid) + + for digit in range(1, 10): + if is_safe(grid, row, column, digit): + grid[row][column] = digit + + if sudoku(grid): + return grid + + grid[row][column] = 0 + + return False + + +def print_solution(grid): + """ + A function to print the solution in the form + of a 9x9 grid + + """ + + for row in grid: + for cell in row: + print(cell, end=" ") + print() + + +if __name__ == "__main__": + + # make a copy of grid so that you can compare with the unmodified grid + for grid in (initial_grid, no_solution): + grid = list(map(list, grid)) + solution = sudoku(grid) + if solution: + print("grid after solving:") + print_solution(solution) + else: + print("Cannot find a solution.")