Sudoku.cpp

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

bool isValid(vector<vector<int>>& board, int row, int col, int val)
{
    for (int i = 0; i < 9; i++) {
        if (board[i][col] == val)
            return false;

        if (board[row][i] == val)
            return false;

        if (board[3 * (row / 3) + i / 3][3 * (col / 3) + i % 3] == val) //Checking corresponding 3x3 grid within the same loop
            return false;
    }
    return true;
}

bool solveSudoku(vector<vector<int>>& board)
{
    for (int i = 0; i < board.size(); i++) {
        for (int j = 0; j < board[0].size(); j++) {
            if (board[i][j] == 0) {
                for (int val = 1; val <= 9; val++) {
                    if (isValid(board, i, j, val)) {
                        board[i][j] = val;

                        if (solveSudoku(board))
                            return true;
                        else
                            board[i][j] = 0;
                    }
                }

                return false;
            }
        }
    }
    return true;
}

int main()
{
    vector<vector<int>> board = { { 3, 0, 0, 0, 0, 1, 0, 0, 2 },
        { 2, 0, 1, 0, 3, 0, 6, 0, 4 },
        { 0, 0, 0, 2, 0, 4, 0, 0, 0 },
        { 8, 0, 9, 0, 0, 0, 1, 0, 6 },
        { 0, 6, 0, 0, 0, 0, 0, 5, 0 },
        { 7, 0, 2, 0, 0, 0, 4, 0, 9 },
        { 0, 0, 0, 5, 0, 9, 0, 0, 0 },
        { 9, 0, 4, 0, 8, 0, 7, 0, 5 },
        { 6, 0, 0, 1, 0, 7, 0, 0, 3 } };

    if (solveSudoku(board)) {
        cout << "Solution : \n";
        for (int i = 0; i < 9; i++) {
            for (int j = 0; j < 9; j++) {
                cout << board[i][j] << " ";
            }
            cout << endl;
        }
    }

    return 0;
}