Solve any sudoku board through a brute force filling algorithm

SudokuSolver.cpp

#include <iostream>
#include <utility>
using namespace std;

// Arrays and macros
#define SIDE_LEN 9
bool rows[SIDE_LEN][SIDE_LEN] = {false};
bool cols[SIDE_LEN][SIDE_LEN] = {false};
bool boxes[SIDE_LEN][SIDE_LEN] = {false};
#define BLANK 0
int grid[SIDE_LEN][SIDE_LEN];
#define BOX(row, col) ((row / 3) * 3 + (col / 3))

// functions
bool recursiveFill(int row, int col);
bool callNext(int row, int col)
{
    col = (col + 1) % SIDE_LEN;
    row = row + (col == 0);
    return recursiveFill(row, col);
}
bool recursiveFill(int row, int col)
{
    if (row == SIDE_LEN)
        return true;
    if (grid[row][col] != BLANK)
        return callNext(row, col);
    for (int i = 0; i < SIDE_LEN; i++)
        if (!rows[i][row] && !cols[i][col] && !boxes[i][BOX(row, col)])
        {
            rows[i][row] = cols[i][col] = boxes[i][BOX(row, col)] = true;
            grid[row][col] = i + 1;
            if (callNext(row, col))
                return true;
            rows[i][row] = cols[i][col] = boxes[i][BOX(row, col)] = false;
        }
    grid[row][col] = BLANK;
    return false;
}

int main(void)
{
    // read given board
    cout << "Enter the board, use " << BLANK << " for a blank tile:" << endl;
    for (int i = 0; i < SIDE_LEN; i++)
        for (int j = 0; j < SIDE_LEN; j++)
        {
            int number;
            cin >> number;
            grid[i][j] = number;
            if (number == BLANK)
                continue;
            rows[number - 1][i] = cols[number - 1][j] =
                boxes[number - 1][BOX(i, j)] = true;
        }
    cout << endl;

    // fill tiles -> print solution
    recursiveFill(0, 0);
    cout << "Solution board:" << endl;
    for (int i = 0; i < SIDE_LEN; i++)
    {
        for (int j = 0; j < SIDE_LEN; j++)
            cout << grid[i][j] << ' ';
        cout << endl;
    }
    return 0;
}