sudoku_empty.py

############################################################
# OPTIONAL HELPER FUNCTIONS
############################################################

def find_next_empty(puzzle):
    # finds the next row, col on puzzle that's not filled yet --> we represent these with -1
    # returns a row, col tuple (or (None, None) if there is none)
    pass

def is_valid(puzzle, guess, row, col):
    # figures out whether the guess at the row/col of the puzzle is a valid guess
    # returns True or False
    pass

############################################################
# SOLVER IMPLEMENTATION
############################################################

def solve_sudoku(puzzle):
    # solve sudoku using backtracking!
    # our puzzle is a list of lists, where each inner list is a row in our sudoku puzzle
    # return solution

    # step 1: choose somewhere on the puzzle to make a guess
    # step 1.1: if there's nowhere left, then we're done because we only allowed valid inputs
    # step 2: if there is a place to put a number, then make a guess between 1 and 9
    # step 3: check if this is a valid guess
    # step 3.1: if this is a valid guess, then place it at that spot on the puzzle
    # step 4: then we recursively call our solver!
    # step 5: it not valid or if nothing gets returned true, then we need to backtrack and try a new number
    # step 6: if none of the numbers that we try work, then this puzzle is UNSOLVABLE!!
    pass