Task: Sudoku
C#: using System;

class SudokuSolver
{
    private int[] grid;

    public SudokuSolver(String s)
    {
        grid = new int[81];
        for (int i = 0; i < s.Length; i++)
        {
            grid[i] = int.Parse(s[i].ToString());
        }
    }

    public void solve()
    {
        try
        {
            placeNumber(0);
            Console.WriteLine("Unsolvable!");
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.Message);
            Console.WriteLine(this);
        }
    }

    public void placeNumber(int pos)
    {
        if (pos == 81)
        {
            throw new Exception("Finished!");
        }
        if (grid[pos] > 0)
        {
            placeNumber(pos + 1);
            return;
        }
        for (int n = 1; n <= 9; n++)
        {
            if (checkValidity(n, pos % 9, pos / 9))
            {
                grid[pos] = n;
                placeNumber(pos + 1);
                grid[pos] = 0;
            }
        }
    }

    public bool checkValidity(int val, int x, int y)
    {
        for (int i = 0; i < 9; i++)
        {
            if (grid[y * 9 + i] == val || grid[i * 9 + x] == val)
                return false;
        }
        int startX = (x / 3) * 3;
        int startY = (y / 3) * 3;
        for (int i = startY; i < startY + 3; i++)
        {
            for (int j = startX; j < startX + 3; j++)
            {
                if (grid[i * 9 + j] == val)
                    return false;
            }
        }
        return true;
    }

    public override string ToString()
    {
        string sb = "";
        for (int i = 0; i < 9; i++)
        {
            for (int j = 0; j < 9; j++)
            {
                sb += (grid[i * 9 + j] + " ");
                if (j == 2 || j == 5)
                    sb += ("| ");
            }
            sb += ('\n');
            if (i == 2 || i == 5)
                sb += ("------+-------+------\n");
        }
        return sb;
    }

    public static void Main(String[] args)
    {
        new SudokuSolver("850002400" +
                         "720000009" +
                         "004000000" +
                         "000107002" +
                         "305000900" +
                         "040000000" +
                         "000080070" +
                         "017000000" +
                         "000036040").solve();
        Console.Read();
    }
}

Python: def initiate():
    box.append([0, 1, 2, 9, 10, 11, 18, 19, 20])
    box.append([3, 4, 5, 12, 13, 14, 21, 22, 23])
    box.append([6, 7, 8, 15, 16, 17, 24, 25, 26])
    box.append([27, 28, 29, 36, 37, 38, 45, 46, 47])
    box.append([30, 31, 32, 39, 40, 41, 48, 49, 50])
    box.append([33, 34, 35, 42, 43, 44, 51, 52, 53])
    box.append([54, 55, 56, 63, 64, 65, 72, 73, 74])
    box.append([57, 58, 59, 66, 67, 68, 75, 76, 77])
    box.append([60, 61, 62, 69, 70, 71, 78, 79, 80])
    for i in range(0, 81, 9):
        row.append(range(i, i+9))
    for i in range(9):
        column.append(range(i, 80+i, 9))

def valid(n, pos):
    current_row = pos/9
    current_col = pos%9
    current_box = (current_row/3)*3 + (current_col/3)
    for i in row[current_row]:
        if (grid[i] == n):
            return False
    for i in column[current_col]:
        if (grid[i] == n):
            return False
    for i in box[current_box]:
        if (grid[i] == n):
            return False
    return True

def solve():
    i = 0
    proceed = 1
    while(i < 81):
        if given[i]:
            if proceed:
                    i += 1
            else:
                i -= 1
        else:
            n = grid[i]
            prev = grid[i]
            while(n < 9):
              if (n < 9):
                  n += 1
              if valid(n, i):
                  grid[i] = n
                  proceed = 1
                  break
            if (grid[i] == prev):
               grid[i] = 0
               proceed = 0
            if proceed:
               i += 1
            else:
               i -=1

def inputs():
    nextt = 'T'
    number = 0
    pos = 0
    while(not(nextt == 'N' or nextt == 'n')):
        print "Enter the position:",
        pos = int(raw_input())
        given[pos - 1] = True
        print "Enter the numerical:",
        number = int(raw_input())
        grid[pos - 1] = number
        print "Do you want to enter another given?(Y, for yes: N, for no)"
        nextt = raw_input()


grid = [0]*81
given = [False]*81
box = []
row = []
column = []
initiate()
inputs()
solve()
for i in range(9):
    print grid[i*9:i*9+9]
raw_input()