# Accepted Python solution

• ``````class Solution:
# @param board, a 9x9 2D array
# Solve the Sudoku by modifying the input board in-place.
# Do not return any value.
def solveSudoku(self, board):
self.board = board
self.solve()

def findUnassigned(self):
for row in range(9):
for col in range(9):
if self.board[row][col] == ".":
return row, col
return -1, -1

def solve(self):
row, col = self.findUnassigned()
#no unassigned position is found, puzzle solved
if row == -1 and col == -1:
return True
for num in ["1","2","3","4","5","6","7","8","9"]:
if self.isSafe(row, col, num):
self.board[row][col] = num
if self.solve():
return True
self.board[row][col] = "."
return False

def isSafe(self, row, col, ch):
boxrow = row - row%3
boxcol = col - col%3
if self.checkrow(row,ch) and self.checkcol(col,ch) and self.checksquare(boxrow, boxcol, ch):
return True
return False

def checkrow(self, row, ch):
for col in range(9):
if self.board[row][col] == ch:
return False
return True

def checkcol(self, col, ch):
for row in range(9):
if self.board[row][col] == ch:
return False
return True

def checksquare(self, row, col, ch):
for r in range(row, row+3):
for c in range(col, col+3):
if self.board[r][c] == ch:
return False
return True
``````

It's a simple backtracking solution.

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