# Ten tips in Solving Sudoku Puzzles

Alvin Ian Chan, 19, was named the Philippines’ new Sudoku grandmaster at the 8th Philippine Super Sudoku Challenge (PSSC). He is currently taking up BS Applied Mathematics majoring in Mathematical Finance.

Here are 10 tips he suggests on how to win at Sudoku:

Sudoku is a popular number placing puzzle wherein you are given a 9x9 grid and your goal is to fill up each grid such that each row, column and 3x3 sub grid contains the numbers from 1 to 9. The rules are straightforward but most of the time you would see yourself staring at the puzzle and wondering how to even start solving the puzzle. Here are 10 tips and tricks in order to solve Sudoku puzzles:

1) **Know the Rules**.

Knowing the rules is like a path in order to reach your destination. For Classical Sudoku – the most popular and simplest variant- each grid should be filled up such that in each row, column and 3x3 sub grid, the numbers from 1 to 9 appear exactly once. Sudoku variants have more or less the same rules as classical Sudoku but with additional or different rules. For example, with diagonal Sudoku, you also need to make sure that the main diagonals contain the numbers 1 to 9. With Irregular Sudoku, instead of making sure that the 3x3 boxes contains the numbers from 1 to 9, you need to make sure that the irregular boundaries(bounded by darker lines) contain the numbers 1 to 9 instead.

2) **Know the Terms**.

Knowing the rules is important but it is not enough. Knowing what terms mean would not only help you understand the rules but would also help you solve Sudoku puzzles. It also helps you understand Sudoku techniques and be able to talk Sudoku with your peers. Some terms used in this article are row (horizontal set of boxes), column (vertical set of boxes) and candidates (numbers that can be correctly placed in that box)

**3) ****Imaginary Lines**

This is actually one of the two ways I start solving puzzles. I first choose a number ( say 1) then I would make imaginary lines in my head like the one in the example below wherein the lines crosses out rows and columns where we can no longer place the number 1**. ** Then I look for boxes where I can safely place the number 1. In the example below it is denoted by the red circles. Notice that in each respective 3x3 box, the box where the red circle lies is the only box where we can safely place the number 1.

**4) ****Filling in the 3x3 boxes **

This is another way of solving the puzzle. Like the header implies, instead of using imaginary lines that covers the whole Sudoku puzzle, focus on 1 3x3 box at a time and fill in the numbers. For example, in the top-right 3x3 box in the puzzle below, place the Red 1 in the box in the second column and first row (can be shorten into 2C1R) since 1 cannot be placed other boxes in the grid. You cannot safely place the numbers 2 and 3 in the 3x3 grid so skip them then write the orange 7 in 1C2R followed by green 9 in 1C1R with the same reasoning as red 1- that you can safely place them in that box since you cannot place them in other box of the 3x3 grid. Go back and write blue 2 in 2C3R and violet 3 in 3C3R since they can now be safely placed their corresponding boxes.

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5) **Notes**

Notes are writing the possible candidates of a cell in the cell. This trick would help you see a clearer picture of the puzzle and helps you to eliminate candidates from other cells. For example, in the illustration below, the upper-right 3x3 box contains notes in its empty boxes while the upper-left 3x3 box contains notes but the number which we can safely say that we cannot place in those boxes are erased. You can now clearly see that we can safely place 3 in 9C2R and 9 in 7C2R

**6) ****Order**

Some Sudoku puzzles can be quite confusing in the sense that you can easily mix up the answer in a box with a different number. There are lots of puzzles that make this mistake when they write number 4 in the box instead of 5. One trick to deal with this is to think either in increasing or decreasing numbers. For example, in tip #4 I look for answers from 1 to 9 and in tip#5 notes are written in increasing order.

**7) ****Look for weakspots**

Every puzzle can be solved in different ways. You can start by filling in the upper right 3x3 box or the upper left 3x3 box. Usually, this would not make any difference but for harder puzzles it would be easier to fill in 3x3 boxes, columns or rows that have fewer empty boxes. For example, a 3x3 box with 2 empty box left is easier to fill in than a 3x3 box with 4 empty box left.

8) **Uniqueness**

When you see a Sudoku puzzle with its corresponding answer key, have you ever wondered if there are other answers aside from the given answer key? Well, most of the time the answer is NO. Most Sudoku puzzles are designed to have one and only answer except on rare circumstances wherein the publisher was not able to check that the puzzle has more than one answer. So, knowing this fact, when it occur to you that there are more than 1 solution in the answer the puzzle you are solving, you should check for errors in the answers that you have placed since most of the time you have already made a mistake.

9) **Research**

There are lots of web resources out there that offer daily puzzles and advanced techniques such as X-Wing and Y-Wing which are more complicated that you might not be able to conceive them on your own.

**10) ****Practice**

Knowing how to solve and being able to solve Sudoku puzzles is not enough. Practice allows you to solve Sudoku puzzles faster and more accurately.

Source of the sudoku puzzles

http://www.sudoku-puzzles.net/