solver.py

board=[
        [7, 8, 0, 4, 0, 0, 1, 2, 0],
        [6, 0, 0, 0, 7, 5, 0, 0, 9],
        [0, 0, 0, 6, 0, 1, 0, 7, 8],
        [0, 0, 7, 0, 4, 0, 2, 6, 0],
        [0, 0, 1, 0, 5, 0, 9, 3, 0],
        [9, 0, 4, 0, 6, 0, 0, 0, 5],
        [0, 7, 0, 3, 0, 0, 0, 1, 2],
        [1, 2, 0, 0, 0, 7, 4, 0, 0],
        [0, 4, 9, 2, 0, 6, 0, 0, 7]
      ]
board2=[
        [5, 3, 0, 0, 7, 0, 0, 0, 0],
        [6, 0, 0, 1, 9, 5, 0, 0, 0],
        [0, 9, 8, 0, 0, 0, 0, 6, 0],
        [8, 0, 0, 0, 6, 0, 0, 0, 3],
        [4, 0, 0, 8, 0, 3, 0, 0, 1],
        [7, 0, 0, 0, 2, 0, 0, 0, 6],
        [0, 6, 0, 0, 0, 0, 2, 8, 0],
        [0, 0, 0, 4, 1, 9, 0, 0, 5],
        [0, 0, 0, 0, 8, 0, 0, 7, 9]
    ]


def print_board(bo):

        for i in range(len(bo)):

                if i%3==0 and i!=0:
                        print("-------------------")

                for j in range(len(bo[0])):

                        if j%3==0 and j!=0:
                                print("|",end="")

                        if j==8:
                                print(bo[i][j])

                        else:
                                print (str(bo[i][j])+" ", end="")


def find_empty(bo):

        for i in range(len(bo)):

                for j in range(len(bo[0])):

                        if bo[i][j]==0:
                                return (i,j) #row,col(ypos,xpos for the board)

        return None


def valid (bo,num,pos):

        #check row
        for i in range(len(bo[0])):

                if bo[pos[0]][i]==num and pos[1]!=i:
                        return False

        #check column
        for i in range(len(bo[0])):

                if bo[i][pos[1]]==num and pos[0]!=i:
                        return False

        #check box

        #getting box index from given index
        box_x=pos[1]//3
        box_y=pos[0]//3

        for i in range(box_y*3,box_y*3+3):#getting index from box index

                for j in range (box_x*3,box_x*3+3):

                        if bo[i][j]==num and (i,j)!=pos:
                                return False

        return True

def solve(bo):

        find=find_empty(bo)#finding empty box

        if not find:#board fully solved
                return True

        else:
                row,col=find

        #backtracking algorithm
        for i in range(1,10):

                if valid(bo,num=i,pos=(row,col)):
                        bo[row][col]=i#valid pos hence insert no.

                        if solve(bo):#solve this new board
                                return True#if board solved return true

                        bo[row][col]=0#else backtrack and set prev value to 0

        return False

print("\n----Given Board---")
print_board(board2)
solve(board2)
print("\n----Solved Board---")
print_board(board2)