# Backtracking | Set 7 (Sudoku)

Given a partially filled 9×9 2D array ‘grid[9][9]’, the goal is to assign digits (from 1 to 9) to the empty cells so that every row, column, and subgrid of size 3×3 contains exactly one instance of the digits from 1 to 9.

## Recommended: Please solve it on “PRACTICE ” first, before moving on to the solution.

Naive Algorithm
The Naive Algorithm is to generate all possible configurations of numbers from 1 to 9 to fill the empty cells. Try every configuration one by one until the correct configuration is found.

Backtracking Algorithm
Like all other Backtracking problems, we can solve Sudoku by one by one assigning numbers to empty cells. Before assigning a number, we check whether it is safe to assign. We basically check that the same number is not present in the current row, current column and current 3X3 subgrid. After checking for safety, we assign the number, and recursively check whether this assignment leads to a solution or not. If the assignment doesn’t lead to a solution, then we try next number for the current empty cell. And if none of the number (1 to 9) leads to a solution, we return false.

```  Find row, col of an unassigned cell
If there is none, return true
For digits from 1 to 9
a) If there is no conflict for digit at row, col
assign digit to row, col and recursively try fill in rest of grid
b) If recursion successful, return true
c) Else, remove digit and try another
If all digits have been tried and nothing worked, return false
```

Following are C++ and Python implementation for Sudoku problem. It prints the completely filled grid as output.

## C/C++

```// A Backtracking program  in C++ to solve Sudoku problem
#include <stdio.h>

// UNASSIGNED is used for empty cells in sudoku grid
#define UNASSIGNED 0

// N is used for the size of Sudoku grid. Size will be NxN
#define N 9

// This function finds an entry in grid that is still unassigned
bool FindUnassignedLocation(int grid[N][N], int &row, int &col);

// Checks whether it will be legal to assign num to the given row, col
bool isSafe(int grid[N][N], int row, int col, int num);

/* Takes a partially filled-in grid and attempts to assign values to
all unassigned locations in such a way to meet the requirements
for Sudoku solution (non-duplication across rows, columns, and boxes) */
bool SolveSudoku(int grid[N][N])
{
int row, col;

// If there is no unassigned location, we are done
if (!FindUnassignedLocation(grid, row, col))
return true; // success!

// consider digits 1 to 9
for (int num = 1; num <= 9; num++)
{
// if looks promising
if (isSafe(grid, row, col, num))
{
// make tentative assignment
grid[row][col] = num;

// return, if success, yay!
if (SolveSudoku(grid))
return true;

// failure, unmake & try again
grid[row][col] = UNASSIGNED;
}
}
return false; // this triggers backtracking
}

/* Searches the grid to find an entry that is still unassigned. If
found, the reference parameters row, col will be set the location
that is unassigned, and true is returned. If no unassigned entries
remain, false is returned. */
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;
}

/* Returns a boolean which indicates whether an assigned entry
in the specified row matches the given number. */
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;
}

/* Returns a boolean which indicates whether an assigned entry
in the specified column matches the given number. */
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;
}

/* Returns a boolean which indicates whether an assigned entry
within the specified 3x3 box matches the given number. */
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;
}

/* Returns a boolean which indicates whether it will be legal to assign
num to the given row,col location. */
bool isSafe(int grid[N][N], int row, int col, int num)
{
/* Check if 'num' is not already placed in current row,
current column and current 3x3 box */
return !UsedInRow(grid, row, num) &&
!UsedInCol(grid, col, num) &&
!UsedInBox(grid, row - row%3 , col - col%3, num);
}

/* A utility function to print grid  */
void printGrid(int grid[N][N])
{
for (int row = 0; row < N; row++)
{
for (int col = 0; col < N; col++)
printf("%2d", grid[row][col]);
printf("\n");
}
}

/* Driver Program to test above functions */
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
printf("No solution exists");

return 0;
}
```

## Python

```# A Backtracking program  in Pyhton to solve Sudoku problem

# A Utility Function to print the Grid
def print_grid(arr):
for i in range(9):
for j in range(9):
print arr[i][j],
print ('n')

# Function to Find the entry in the Grid that is still  not used
# Searches the grid to find an entry that is still unassigned. If
# found, the reference parameters row, col will be set the location
# that is unassigned, and true is returned. If no unassigned entries
# remain, false is returned.
# 'l' is a list  variable that has been passed from the solve_sudoku function
# to keep track of incrementation of Rows and Columns
def find_empty_location(arr,l):
for row in range(9):
for col in range(9):
if(arr[row][col]==0):
l[0]=row
l[1]=col
return True
return False

# Returns a boolean which indicates whether any assigned entry
# in the specified row matches the given number.
def used_in_row(arr,row,num):
for i in range(9):
if(arr[row][i] == num):
return True
return False

# Returns a boolean which indicates whether any assigned entry
# in the specified column matches the given number.
def used_in_col(arr,col,num):
for i in range(9):
if(arr[i][col] == num):
return True
return False

# Returns a boolean which indicates whether any assigned entry
# within the specified 3x3 box matches the given number
def used_in_box(arr,row,col,num):
for i in range(3):
for j in range(3):
if(arr[i+row][j+col] == num):
return True
return False

# Checks whether it will be legal to assign num to the given row,col
#  Returns a boolean which indicates whether it will be legal to assign
#  num to the given row,col location.
def check_location_is_safe(arr,row,col,num):

# Check if 'num' is not already placed in current row,
# current column and current 3x3 box
return not used_in_row(arr,row,num) and not used_in_col(arr,col,num) and not used_in_box(arr,row - row%3,col - col%3,num)

# Takes a partially filled-in grid and attempts to assign values to
# all unassigned locations in such a way to meet the requirements
# for Sudoku solution (non-duplication across rows, columns, and boxes)
def solve_sudoku(arr):

# 'l' is a list variable that keeps the record of row and col in find_empty_location Function
l=[0,0]

# If there is no unassigned location, we are done
if(not find_empty_location(arr,l)):
return True

# Assigning list values to row and col that we got from the above Function
row=l[0]
col=l[1]

# consider digits 1 to 9
for num in range(1,10):

# if looks promising
if(check_location_is_safe(arr,row,col,num)):

# make tentative assignment
arr[row][col]=num

# return, if sucess, ya!
if(solve_sudoku(arr)):
return True

# failure, unmake & try again
arr[row][col] = 0

# this triggers backtracking
return False

# Driver main function to test above functions
if __name__=="__main__":

# creating a 2D array for the grid
grid=[[0 for x in range(9)]for y in range(9)]

# assigning values to the grid
grid=[[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 sucess print the grid
if(solve_sudoku(grid)):
print_grid(grid)
else:
print "No solution exists"

# The above code has been contributed by Harshit Sidhwa.

```

Output:

```  3 1 6 5 7 8 4 9 2
5 2 9 1 3 4 7 6 8
4 8 7 6 2 9 5 3 1
2 6 3 4 1 5 9 8 7
9 7 4 8 6 3 1 2 5
8 5 1 7 9 2 6 4 3
1 3 8 9 4 7 2 5 6
6 9 2 3 5 1 8 7 4
7 4 5 2 8 6 3 1 9
```