A cool algorithm to check a Sudoku field?

Question

Does anyone know a simple algorithm to check if a Sudoku-Configuration is valid? The simplest algorithm I came up with is (for a board of size n) in Pseudocode

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Answer 1

array = [1,2,3,4,5,6,7,8,9]  
sudoku = int [][]
puzzle = 9 #9x9
columns = map []
units = map [] # box    
unit_l = 3 # box width/height
check_puzzle()    


def strike_numbers(line, line_num, columns, units, unit_l):
    count = 0
    for n in line:
        # check which unit we're in
        unit = ceil(n / unit_l) + ceil(line_num / unit_l) # this line is wrong - rushed
        if units[unit].contains(n): #is n in unit already?
             return columns, units, 1
        units[unit].add(n)
        if columns[count].contains(n): #is n in column already?
            return columns, units, 1
        columns[count].add(n)
        line.remove(n) #remove num from temp row
    return columns, units, line.length # was a number not eliminated?

def check_puzzle(columns, sudoku, puzzle, array, units):
    for (i=0;i< puzzle;i++):
        columns, units, left_over = strike_numbers(sudoku[i], i, columns, units) # iterate through rows
        if (left_over > 0): return false

Without thoroughly checking, off the top of my head, this should work (with a bit of debugging) while only looping twice. O(n^2) instead of O(3(n^2))