Python - Homework3 -Sudoku - Solved

## Introduction
In this homework, you will need to design a sudoku solver that can automatically solve the sudoku puzzle within reasonable time constraint.<br>
A sudoku puzzle is represented by a 9x9 matrix with some missing elements in it. Each element in the matrix is an integer value from 1 to 9.
```
For example:
        columns
                                 +-----+ +-----+ +-----+
   9,5,7,6,1,3,2,8,4             |9,5,7| |6,1,3| |2,8,4|
   4,8,3,2,5,7,1,9,6             |4,8,3| |2,5,7| |1,9,6|
r  6,1,2,8,4,9,5,3,7             |6,1,2| |8,4,9| |5,3,7|
o  1,7,8,3,6,4,9,5,2   ------->  +-----+ +-----+ +-----+
w  5,2,4,9,7,1,3,6,8   ------->  block0  block1  block2
s  3,6,9,5,2,8,7,4,1
   8,4,5,7,9,2,6,1,3             +-----+ +-----+ +-----+
   2,9,1,4,3,6,8,7,5             |1,7,8| |3,6,4| |9,5,2|
   7,3,6,1,8,5,4,2,9             |5,2,4| |9,7,1| |3,6,8|
                                 |3,6,9| |5,2,8| |7,4,1|
                                 +-----+ +-----+ +-----+
                                 block3  block4  block5
                                 
                                 +-----+ +-----+ +-----+
                                 |8,4,5| |7,9,2| |6,1,3|
                                 |2,9,1| |4,3,6| |8,7,5|
                                 |7,3,6| |1,8,5| |4,2,9|
                                 +-----+ +-----+ +-----+
                                 block6  block7  block8
```
A solved sudoku puzzle should satisfy following requirements:
1. Each row consists of a sequence of numbers from 1 to 9 ,and each digit can only occurs once
2. Each col consists of a sequence of numbers from 1 to 9 ,and each digit can only occurs once
3. Each block consists of a sequence of numbers from 1 to 9 ,and each digit can only occurs once

## Requirements
Given the template code provided by TA, you need to fulfill all the methods and functions in the code. The score/point for each method or function is explained in the docstring.
#### MUST READ
+ You cannot use any third-party package such as numpy, pandas, and etc.
+ You can only use python primitive types and statements to solve the problem.
+ Do not copy others code. (0 scores for punishment)

## Expected Execution Result
### __ init __
```python
# Content in sudoku2.txt
# ======================
# 0,5,7,6,1,3,2,8,4
# 4,8,3,2,5,0,0,0,6
# 6,1,2,8,4,0,5,0,7
# 1,7,8,3,0,0,0,0,2
# 5,2,4,9,7,1,3,6,8
# 3,6,0,0,0,0,7,4,1
# 8,4,5,7,9,2,6,1,3
# 2,9,1,4,3,6,8,7,5
# 7,3,6,1,8,0,0,0,0

# Test constructor
sudoku1=Sudoku("sudoku2.txt")
```
### __ str __
```python
# Test __str__ magic method
print(sudoku1, end="\n\n")
```
0  5  7  6  1  3  2  8  4<br>
4  8  3  2  5  0  0  0  6<br>
6  1  2  8  4  0  5  0  7<br>
1  7  8  3  0  0  0  0  2<br>
5  2  4  9  7  1  3  6  8<br>
3  6  0  0  0  0  7  4  1<br>
8  4  5  7  9  2  6  1  3<br>
2  9  1  4  3  6  8  7  5<br>
7  3  6  1  8  0  0  0  0

### check_block
```python
# Test check_block
print(sudoku1.check_block(0))
print(sudoku1.check_block(3))
print(sudoku1.check_block(6))
```
True<br>
True<br>
True

### check_row
```python
# Test check_row
print(sudoku1.check_row(0))
print(sudoku1.check_row(1))
print(sudoku1.check_row(2))
```
True<br>
True<br>
True

### check_col
```python
# Test check_col
print(sudoku1.check_col(0))
print(sudoku1.check_col(1))
print(sudoku1.check_col(2))
```
True<br>
True<br>
True

### is_solved
```python
#Test is_solved
print(sudoku1.is_solved())
```
False

### solve
```python
# Test solve
sudoku1.solve()

print(sudoku1, end="\n\n")
```
9  5  7  6  1  3  2  8  4<br>
4  8  3  2  5  1  9  6  6<br>
6  1  2  8  4  9  5  3  7<br>
1  7  8  3  6  4  9  5  2<br>
5  2  4  9  7  1  3  6  8<br>
3  6  9  5  2  8  7  4  1<br>
8  4  5  7  9  2  6  1  3<br>
2  9  1  4  3  6  8  7  5<br>
7  3  6  1  8  5  4  2  9

### is_solved after solve
```python
#Test is_solved
print(sudoku1.is_solved())
```
True