solution.h

#include <vector>
using std::vector;
#include <unordered_map>
#include <unordered_set>
#include <iostream>

class Solution {
    bool findEmptyCell(const vector<vector<char> > &board, size_t &row, size_t &col)
    {
        for (row = 0; row < board.size(); ++row)
            for (col = 0; col < board[row].size(); ++col)
                if (board[row][col] == '.') return true;
        return false;
    }
    bool isSafe(const vector<vector<char> > &board, size_t row, size_t col, char c)
    {
        for (size_t i=0; i<9; ++i) {
            if (board[row][i] == c) return false;
            if (board[i][col] == c) return false;
            if (board[row/3 * 3 + i / 3][col/3 * 3 + i % 3] == c) return false;
        }
        return true;
    }
public:
    bool solveSudoku(vector<vector<char> > &board) {
        size_t row = 0, col = 0;
        if (!findEmptyCell(board, row, col)) return true;
        for (char c : {'1', '2', '3', '4', '5', '6', '7', '8', '9'})
            if (isSafe(board, row, col, c)) {
                board[row][col] = c;
                if (solveSudoku(board)) return true;
                board[row][col] = '.';
            }
        return false;
    }
};