Create Sudoku_solver_backtracking.py

Backtracking/Sudoku Solver/Sudoku_solver_backtracking.py

@@ -0,0 +1,153 @@
+""" 
+    Author : Mohit Kumar
+
+    9*9 Sudoku Solver in python using Backtracking 
+
+    Given a partially filled 9×9 2D array, the objective is to fill a 9×9
+    square grid with digits numbered 1 to 9, so that every row, column, and
+    and each of the nine 3×3 sub-grids contains all of the digits.
+
+    In This Solver we try filling digits one by one. Whenever we find that current 
+    digit cannot lead to a solution, we remove it (backtrack) and try next digit. 
+    This is better than naive approach (generating all possible combinations of digits
+    and then trying every combination one by one as it drops a set of permutations
+    whenever it backtracks.
+
+
+"""
+from typing import List, Tuple
+
+Table = List[List[int]]  # typing hint for board
+
+# intial values of sudoku board
+# 0 represents place to be filled
+
+board = [
+    [5, 0, 0, 0, 0, 0, 0, 0, 0],
+    [3, 4, 0, 1, 0, 0, 0, 0, 7],
+    [0, 9, 0, 0, 0, 6, 0, 0, 0],
+    [6, 0, 0, 0, 2, 0, 0, 0, 9],
+    [0, 0, 4, 9, 8, 0, 1, 0, 0],
+    [0, 1, 0, 0, 4, 0, 0, 0, 0],
+    [0, 0, 0, 0, 0, 3, 0, 2, 6],
+    [0, 0, 0, 0, 0, 0, 8, 0, 0],
+    [9, 0, 0, 0, 0, 0, 3, 4, 0],
+]
+
+
+def find_empty(board: Table) -> Tuple[int, int]:
+    """
+    This function finds the empty position
+    to be filled
+
+    Parameters :
+        Input : Sudoku Table
+        Output : Tuple[int,int]
+                 value of row and column of empty position
+
+    """
+    for i, row in enumerate(board):
+        for j, val in enumerate(row):
+            if val == 0:
+                return (i, j)
+
+
+def valid(board: Table, num: int, pos: Tuple[int, int]) -> bool:
+    """
+    This function checks the Table to see if each row,
+    column, and the 3x3 subgrids contain the digit 'num.
+    It returns False if it is not 'safe' (a duplicate digit
+    is found) else returns True if it is 'safe'
+
+    Parameters :
+    Input : Board : Table
+            num : int
+            pos : Tuple[int, int]
+                  which contain(row, column) of pos
+    """
+
+    for i in range(len(board)):
+        # this loop checks number in same row and column
+        if board[pos[0]][i] == num or board[i][pos[1]] == num:
+            return False
+
+    # these are the row and column value for 3*3 subgrid
+    # in which the num belong
+    cur_row = 3 * (pos[0] // 3)
+    cur_col = 3 * (pos[1] // 3)
+
+    # this loop check for the num present in 3*3 subgrid
+    for i in range(cur_row, cur_row + 3):
+        for j in range(cur_col, cur_col + 3):
+            if board[i][j] == num:
+                return False
+    return True
+
+
+def solve(board: Table) -> bool:
+    """
+    This is Solver function which uses Backtracking
+    it repeatedly fills values in empty spaces and if filled
+    value is wrong it backtracks to last value and start filling again
+    with diffrent value
+
+    Parameteres :
+    Input : Board : Table
+
+    Output : True if it solves the Sudoku Table
+             False if Sudoku Table is incorrect
+
+    """
+    # finds the empty position to be filled
+    find = find_empty(board)
+
+    if not find:
+
+        # if no empty position is there
+        # we solved sudoku and returns True
+        return True
+    else:
+        # row and column value for the corresponding empty position
+        row, col = find
+
+    # this loop fills the value from 1- 10 to empty position
+    for i in range(1, 10):
+
+        # this checks if corresponding filled value is correct or not
+        if valid(board, i, (row, col)):
+            board[row][col] = i
+
+            if solve(board):
+                # if value is correct then moves to next value
+                return True
+
+            # if value is not correct mark this as empty
+            board[row][col] = 0
+
+    # backtracks to previous filled value
+    return False
+
+
+def print_board(board: Table) -> None:
+    """
+    This function prints Board  in form of
+    9*9 matrix
+    """
+    for i in range(len(board)):
+        if i % 3 == 0 and i > 0:
+            print("- - - - - - - - - - - - - - -  ")
+        for j in range(len(board[i])):
+            if j % 3 == 0 and j > 0:
+                print("| ", end="")
+            print(board[i][j], " ", end="")
+            if j == len(board[i]) - 1:
+                print("\n")
+
+
+print("Before Solving Sudoku : \n")
+print_board(board)
+if solve(board):
+    print("\n********After Solving *********\n")
+    print_board(board)
+else:
+    print("Enter Valid 3*3 sudoku pattern\n")