Rules and how to solve Sudoku.
Rules
  1. Fill the empty white cells with numbers 1 to 9 to meet the following conditions.
  2. The same number is not included in the same horizontal line, including the cells given in the problem. Numbers from 1 to 9 enter once.
  3. The same number is not included in the same vertical column, including the cell given in the problem. Numbers from 1 to 9 enter once.
  4. The same numbers are not included in the 3 × 3 blocks surrounded by thick lines, including the cells given in the question. Numbers from 1 to 9 enter once.
  5. When the total number of squares is 25 × 25, the numbers used are 1 to 25, and the blocks are 5 × 5. Similarly, if the whole is 16x16, the numbers to use are 1-16, and the block is 4x4. The same applies to other number of cells.


jump to Sudoku problem.
Tips for solving
解くポイントは問題で与えられている枡を含め既に確定している枡に注目します。 その数値のある同一横行/縦列/ブロックにはもうその数字は入りません。
ある横行を考えた時には、縦列やブロックからの制限により特定の数字を入れられない場所が在ります。 ある縦列を考えた時には、横行やブロックからの制限により特定の数字を入れられない場所が在ります。 同じようにあるブロックを考えた時には、横行や縦列からの制限により特定の数字を入れられない場所が在ります。
既に確定した同じ数字が2個所あれば両方からその数字が入らない場所を消していくとだんだんと入る場所を限定していけます。 このように限定していってある数字が同一横行/縦列/ブロックの1個所にしか入れられなくなればその枡に確定できます。
The points to be solved focus on the cells already determined, including the cells given in the problem. The same row / column / block with that number no longer contains that number.
Given a row, there are places where certain numbers cannot be entered due to restrictions from columns and blocks. Given a column, there are places where certain numbers cannot be entered due to restrictions from rows or blocks. Similarly, when considering a certain block, there are places where specific numbers cannot be entered due to restrictions from rows and columns.
If there are two places where the same number has already been confirmed, you can gradually limit the places where you can enter by deleting places where the number does not enter from both places. If a number can only be entered in one place in the same row / column / block by these restrictions, it can be confirmed to that cell.
The following figure is an example. Here is an example of 9x9, so the numbers used are 1-9. Each block is a 3x3 square surrounded by thick lines. For simplicity, the background color of the problem cell is gray and thick with numbers. ex0
First look at the upper left block.The first line has 9 in the third cell from the right, and the second line has 9 in the fifth cell from the left. Therefore, in this block, 9 can only be entered in the third row, but the only empty cell in the third row is the cell with the second blue frame from the left. You can only enter 9 in this cell, the number of this cell is determined.
Similarly, paying attention to the green arrow 4, in the center block on the left end, only 4 can be inserted in the green framed cell.
In the same way, if you enter numbers that can only be entered in one place of columns, rows and blocks, you should be able to solve until the next figure.
ex1
Next, look at the numbers in the red framed cell below the number 3 in the fourth cell from the top in the rightmost column.
The numbers in the same vertical column (1,3,5,2) ,the numbers in the same horizontal row (6,7,8,9) and the numbers in the same block (3,6,7) are not placed on this red framed cell, there is only 4 to enter after all.
ex1
By using the newly written numbers and thinking in the same way as above, you should be able to solve the following figure.(Stopped here for explanation.) ex2
You can solve this example by repeating the previous steps, but let's look at a different approach.
The blue framed cell in the middle block where the number is not determined contains 1, 2, 4, or 6. This row already contains 4 for the second from the left and 6 for the second from the right, so only 1 or 2 is entered.
One of 1,2,4,6 is also entered in the green framed cell, but 1,2 is entered in the blue framed cell, so 4 or 6 is entered here.
Considering the red framed cell, there are only 1 and 9 numbers left in a row, so 1 or 9 will be entered. For difficult problems, it is sometimes useful to write down the candidate numbers.
ex3
In the previous figure, there is 1 above the red frameed cell on the right, so it is 9 and the cell in the red framed cell on the left is 1.
Similarly, there is 1 above the blue framed cell on the left, so this cell is 2 and the cell with the blue framed cell on the right is 1.
Here is another way of thinking.
ex4
Let's look at the middle block at the bottom. The middle row of this block already contains 5,8,3. There is 4 in the cell to the left of the cell in the green frameed cell in the row above it, so 4 does not enter in the green framed cell. 4 goes into one of the open blue framed cell in the bottom row. Considering the block at the bottom right, 4 will be in any of the blue frameed cell, so 4 will only be in the middle row. In the rightmost column, 4 is in the fifth cell from the top, so in the rightmost block, 4 will be in one of the red framed cells. ex5
There are other approaches, but if you fill in the numbers in order, you will eventually reach the answer in the following figure. ex6


2020.2.28 Modified
2010.6.11 First edition
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