sudoko.cpp

#include <bits/stdc++.h>
using namespace std;
#define UNASSIGNED 0
#define N 9
bool FindUnassignedLocation(int grid[N][N],
							int& row, int& col);

bool isSafe(int grid[N][N], int row,
			int col, int num);

bool SolveSudoku(int grid[N][N])
{
	int row, col;
	if (!FindUnassignedLocation(grid, row, col))
		// success!
		return true;
	for (int num = 1; num <= 9; num++)
	{
		
		// Check if looks promising
		if (isSafe(grid, row, col, num))
		{
			
			// Make tentative assignment
			grid[row][col] = num;

			// Return, if success
			if (SolveSudoku(grid))
				return true;

			// Failure, unmake & try again
			grid[row][col] = UNASSIGNED;
		}
	}
	return false;
}
bool FindUnassignedLocation(int grid[N][N],
							int& row, int& col)
{
	for (row = 0; row < N; row++)
		for (col = 0; col < N; col++)
			if (grid[row][col] == UNASSIGNED)
				return true;
	return false;
}
bool UsedInRow(int grid[N][N], int row, int num)
{
	for (int col = 0; col < N; col++)
		if (grid[row][col] == num)
			return true;
	return false;
}

bool UsedInCol(int grid[N][N], int col, int num)
{
	for (int row = 0; row < N; row++)
		if (grid[row][col] == num)
			return true;
	return false;
}

bool UsedInBox(int grid[N][N], int boxStartRow,
			int boxStartCol, int num)
{
	for (int row = 0; row < 3; row++)
		for (int col = 0; col < 3; col++)
			if (grid[row + boxStartRow]
					[col + boxStartCol] ==
									num)
				return true;
	return false;
}

bool isSafe(int grid[N][N], int row,
			int col, int num)
{
	return !UsedInRow(grid, row, num)
		&& !UsedInCol(grid, col, num)
		&& !UsedInBox(grid, row - row % 3,
						col - col % 3, num)
		&& grid[row][col] == UNASSIGNED;
}
void printGrid(int grid[N][N])
{
	for (int row = 0; row < N; row++)
	{
		for (int col = 0; col < N; col++)
			cout << grid[row][col] << " ";
		cout << endl;
	}
}

// Driver Code
int main()
{
	// 0 means unassigned cells
	int grid[N][N] = { { 3, 0, 6, 5, 0, 8, 4, 0, 0 },
					{ 5, 2, 0, 0, 0, 0, 0, 0, 0 },
					{ 0, 8, 7, 0, 0, 0, 0, 3, 1 },
					{ 0, 0, 3, 0, 1, 0, 0, 8, 0 },
					{ 9, 0, 0, 8, 6, 3, 0, 0, 5 },
					{ 0, 5, 0, 0, 9, 0, 6, 0, 0 },
					{ 1, 3, 0, 0, 0, 0, 2, 5, 0 },
					{ 0, 0, 0, 0, 0, 0, 0, 7, 4 },
					{ 0, 0, 5, 2, 0, 6, 3, 0, 0 } };
	if (SolveSudoku(grid) == true)
		printGrid(grid);
	else
		cout << "No solution exists";

	return 0;
}