Task: Sudoku
C: #include <stdio.h>

void show(int *x)
{
	int i, j;
	for (i = 0; i < 9; i++) {
		if (!(i % 3)) putchar('\n');
		for (j = 0; j < 9; j++)
			printf(j % 3 ? "%2d" : "%3d", *x++);
		putchar('\n');
	}
}

int trycell(int *x, int pos)
{
	int row = pos / 9;
	int col = pos % 9;
	int i, j, used = 0;

	if (pos == 81) return 1;
	if (x[pos]) return trycell(x, pos + 1);

	for (i = 0; i < 9; i++)
		used |= 1 << (x[i * 9 + col] - 1);

	for (j = 0; j < 9; j++)
		used |= 1 << (x[row * 9 + j] - 1);

	row = row / 3 * 3;
	col = col / 3 * 3;
	for (i = row; i < row + 3; i++)
		for (j = col; j < col + 3; j++)
			used |= 1 << (x[i * 9 + j] - 1);

	for (x[pos] = 1; x[pos] <= 9; x[pos]++, used >>= 1)
		if (!(used & 1) && trycell(x, pos + 1)) return 1;

	x[pos] = 0;
	return 0;
}

void solve(const char *s)
{
	int i, x[81];
	for (i = 0; i < 81; i++)
		x[i] = s[i] >= '1' && s[i] <= '9' ? s[i] - '0' : 0;

	if (trycell(x, 0))
		show(x);
	else
		puts("no solution");
}

int main(void)
{
	solve(	"5x..7...."
		"6..195..."
		".98....6."
		"8...6...3"
		"4..8.3..1"
		"7...2...6"
		".6....28."
		"...419..5"
		"....8..79"	);

	return 0;
}

C++: #include <iostream>
using namespace std;

class SudokuSolver {
private:
    int grid[81];

public:

    SudokuSolver(string s) {
        for (unsigned int i = 0; i < s.length(); i++) {
            grid[i] = (int) (s[i] - '0');
        }
    }

    void solve() {
        try {
            placeNumber(0);
            cout << "Unsolvable!" << endl;
        } catch (char* ex) {
            cout << ex << endl;
            cout << this->toString() << endl;
        }
    }

    void placeNumber(int pos) {
        if (pos == 81) {
            throw (char*) "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;
            }
        }
    }

    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;
    }

    string toString() {
        string sb;
        for (int i = 0; i < 9; i++) {
            for (int j = 0; j < 9; j++) {
                char c[2];
                c[0] = grid[i * 9 + j] + '0';
                c[1] = '\0';
                sb.append(c);
                sb.append(" ");
                if (j == 2 || j == 5)
                    sb.append("| ");
            }
            sb.append("\n");
            if (i == 2 || i == 5)
                sb.append("------+-------+------\n");
        }
        return sb;
    }

};

int main() {
    SudokuSolver ss("850002400"
                    "720000009"
                    "004000000"
                    "000107002"
                    "305000900"
                    "040000000"
                    "000080070"
                    "017000000"
                    "000036040");
    ss.solve();
    return EXIT_SUCCESS;
}