Solving Tips

How to Use "Guessing" in Sudoku? From Intuition to Logical Trial

2025-01-23 · 8 min read

In the Sudoku community, "guessing" has always been a controversial topic. Some believe that guessing is "cheating," and that real experts don't need to guess; others think guessing is a necessary means to solve puzzles. So, should you use guessing? How can you use it "skillfully"?

                 Core Message of This Article:

                Guessing isn't about random trial and error—it's about logical exploration. When you master the right approach, "guessing" is actually "hypothesis testing"—a perfectly valid solving strategy.

What is "Guessing"?

In Sudoku, "guessing" typically means: when you can't find a definite solution, you assume a certain cell contains a certain number, then continue reasoning to see if it leads to a contradiction.

Type	Description	Recommended?
Random Guessing	Pick whatever looks right, try another if wrong	Not Recommended
Intuitive Trial	Based on experience, try candidates with higher probability	Acceptable
Hypothesis Testing	Strategically select cells and candidates, then verify through reasoning	Recommended

When Should You NOT Guess?

Important Principle:
Guessing should be your last resort, not your first reaction. Before guessing, make sure you have:

Used Naked Singles to scan all cells
Used Hidden Singles to check all rows, columns, and boxes
Tried Naked Pairs, Naked Triples, and other intermediate techniques
For harder puzzles, tried X-Wing, XY-Wing, and other advanced techniques

Many players think they need to guess when they've simply missed a hidden logical solution. Standard Sudoku puzzles are guaranteed to have a unique solution that can theoretically be solved through pure logic.

When Can You Guess?

Consider using a guessing strategy in these situations:

1 Can't find a logical solution — You've checked repeatedly and confirmed no techniques are overlooked

2 Competition or timed challenge — Time is running out, guessing might be more efficient than searching

3 Non-standard puzzles — Some variant Sudokus or flawed puzzles may require trial and error

4 Learning and verification — Want to confirm whether a candidate works, using trial to understand puzzle structure

How to Guess "Logically"?

Strategy 1: Choose Bi-Value Cells

The best starting point for guessing is cells with only two candidates. The reason is simple:

Only two possibilities, 50% success rate
If wrong, the other must be correct
Short reasoning chain, easy to find contradictions

Example:
Suppose cell 5E has candidates {3, 7}

Steps:
1. Assume 5E = 3
2. Continue reasoning based on this assumption
3. If contradiction found → 5E = 7 is the correct answer
4. If no contradiction → Continue solving (but can't be 100% sure 3 is correct)

Strategy 2: Choose Key Positions

Prioritize cells that have significant impact on the puzzle:

Intersection points: Cells affecting multiple rows, columns, and boxes
Rare numbers: Positions where that number appears less frequently
Bottleneck areas: Regions with few empty cells, where filling one triggers a chain reaction

Strategy 3: Record and Backtrack

Practical Tips:
When solving on paper, use a pencil to mark assumed numbers, or use different colors/symbols to distinguish them. This makes it easy to erase and backtrack when you find a contradiction.

On electronic devices, many apps have save/snapshot features—save your state before guessing.

Advanced: Bifurcation Method

Bifurcation is a systematic guessing method, similar to how computers solve Sudoku:

1 Choose a branch point — Find a bi-value cell, call it A = {x, y}

2 Create branches — Branch 1 assumes A=x, Branch 2 assumes A=y

3 Deep reasoning — In Branch 1, advance as far as possible using all logical techniques

4 Evaluate results — If Branch 1 produces a contradiction, Branch 2 is correct; if Branch 1 solves the puzzle, done!

Note:
Bifurcation can be nested (branches within branches), which causes exponential complexity growth. If you find yourself needing multiple levels of nesting, you've probably missed a logical technique—go back and check first.

Improving Guessing Efficiency

Technique	Explanation
Start with simple reasoning	After assuming, use simple techniques (singles, elimination) to quickly advance—easier to find contradictions
Focus on same row/column/box	The assumption's impact first propagates to cells in the same row, column, and box
Look for chain reactions	If the assumption causes a cell to become bi-value or solved, keep tracing
Recognize contradiction signs	Duplicate numbers in same region, or cell with no candidates = contradiction

Summary: Turn Guessing into Reasoning

                 Key Points:

                Guessing is a backup plan—prioritize logical techniques
Choose bi-value cells as your starting point
Keep good records for easy backtracking
After assuming, continue with normal logic—don't chain-guess
When you find a contradiction, backtrack immediately and confirm the other option

            

When you "guess" using this method, you're actually performing hypothesis testing—a completely valid logical reasoning method. Mathematicians and scientists use this approach every day!

So don't worry about whether "guessing is cheating." The key is: are you randomly guessing for luck, or are you making strategic trials? The latter is absolutely part of advanced solving techniques.

Start Practicing:
Click here to start a Sudoku game and try applying these guessing strategies when you encounter difficulties!

All Techniques Back to Article List