I've implemented backtracking algorithm in python to solve a sudoku. During the steps of the algorithm I check, if the sudoku is already solved and when it is, I want to return it. But my function just returns None. Can someone explain me the reason for this and give me a hint for the solution?
#Test-Sudoku
grid = [[1,0,7,0,0,5,4,0,0],
[9,0,0,3,7,0,0,0,0],
[0,2,3,0,8,0,0,0,0],
[0,9,2,0,0,0,0,7,0],
[0,7,0,0,6,0,0,1,0],
[0,6,0,0,0,0,8,9,0],
[0,0,0,0,4,0,3,6,0],
[0,0,0,0,3,7,0,0,1],
[0,0,8,2,0,0,5,0,7]]
def possible(row,col,num,grid):
""" Check if num can be passed to grid[row][col]"""
if grid[row][col] != 0:
return False
for i in range(9):
if grid[row][i] == num or grid[i][col] == num:
return False
row0 = (row//3)*3
col0 = (col//3)*3
for i in range(3):
for j in range(3):
if grid[row0+i][col0+j] == num:
return False
return True
def is_solved(grid):
""" Check if Sudoku is already solved/full"""
for i in range(9):
for j in range(9):
if grid[i][j] == 0:
return False
return True
def solve(grid):
""" Backtracking algorithm to solve Sudoku"""
for r in range(9):
for c in range(9):
if grid[r][c] == 0:
for i in range(1,10):
if possible(r, c, i, grid):
grid[r][c] = i
if is_solved(grid):
# Print grid for test
print("Show Solved Sudoku:")
print(grid)
return(grid)
solve(grid)
grid[r][c] = 0
return
return
solved_sudoku = solve(grid)
print(solved_sudoku)
because you use return which returns none
you store the result in grid and grid changes in every turn
try this:
def solve(grid):
""" Backtracking algorithm to solve Sudoku"""
for r in range(9):
for c in range(9):
if grid[r][c] == 0:
for i in range(1,10):
if possible(r, c, i, grid):
grid[r][c] = i
solve(grid)
if is_solved(grid):
return
grid[r][c] = 0
return
solve(grid)
print(grid)