How to Play Sudoku

Sudoku looks like a maths puzzle, but there's no adding involved and the numbers could just as easily be nine colours or nine animals. It's a puzzle about placement. Here's everything you need to start.

The board

A Sudoku grid is nine rows by nine columns — eighty-one cells in total. Those cells are also carved into nine smaller 3×3 squares, usually drawn with thicker borders. People call those squares "boxes" or "regions." Rows, columns, and boxes are the three kinds of group you'll care about, and Sudoku folk lump them together under one word: units.

When you open a puzzle, some cells already hold numbers. Those are the givens (or "clues"), and they never change — they're the anchor the whole solution hangs off. Your job is to fill in every empty cell.

The one rule

Honestly, there's just one, and it repeats three times: each row, each column, and each 3×3 box must contain the digits 1 through 9, with nothing repeated. That's it. No digit appears twice in the same row. None appears twice in the same column. And none shows up twice inside the same box.

Because every unit holds nine cells and nine different digits, "use each digit once" and "leave no gaps" turn out to be the same statement. A finished grid has all nine digits in every row, every column, and every box — no more, no less.

How to actually start

Beginners often freeze at the empty cells. Don't. Look at the numbers that are already there and ask the opposite question: not "what goes in this empty cell?" but "where can this digit not go?"

Pick a digit that appears a lot in the givens — say there are already five 7s on the board. Each of those 7s forbids any other 7 in its row, its column, and its box. Sketch those forbidden lines in your head and you'll often find a box where only a single empty cell is left standing for the 7. That cell must be a 7. This trick — scanning a digit across the grid — is called cross-hatching, and it'll carry you through every easy puzzle.

A quick example: suppose the top-left box still needs a 4, and you can see a 4 sitting in its top row and another in its middle row (both outside the box, but on those lines). Two of the box's three rows are now off-limits for 4. If the bottom row of that box has just one empty cell, the 4 has nowhere else to go. Placed.

Pencil marks

Once the obvious placements dry up, switch to candidates — small numbers you jot into a cell to track which digits are still legal there. A cell that can only be a 2 or a 5 gets a tiny "2 5" in the corner. Pencil marks turn a vague grid into a map: when a cell's list shrinks to one number, you've found a placement, and when a digit can only land in one spot of a unit, same thing. Most of the named techniques are really just clever ways of crossing candidates off.

On this site you can fill candidates yourself in notes mode, and the hint button always explains why a candidate can be removed rather than just doing it for you.

Mistakes newcomers make

Guessing. A proper Sudoku never needs a guess. If you're tempted to guess, there's almost always a logical step you've missed — slow down and scan again.
Forgetting the box. People watch the rows and columns and overlook the 3×3 box. The box constraint is often the one that cracks a cell open.
Not updating pencil marks. Every time you place a digit, it should disappear from the candidates of its row, column, and box. Stale marks lead to stale conclusions.

That's genuinely all

One rule, three units, no arithmetic. Everything else — the elegant techniques, the satisfying chains — grows out of that single constraint. When you're ready to put it to use, start a puzzle, and when easy grids stop being a challenge, the solving techniques guide is the next step.

数独怎么玩

数独看着像数学题，其实根本不用算加减，那九个数字换成九种颜色、九种动物也照样成立。它考的是「摆位置」。下面是你上手要知道的全部。

盘面

数独的盘是九行乘九列——一共八十一格。这些格子又被划成九个更小的 3×3 方块，通常用更粗的线框出来，大家管它们叫「宫」或「区域」。行、列、宫，是你要操心的三类组合，数独圈里把它们统称为一个词：单元。

打开一道题时，有些格子里已经有数字了。这些叫初始数字（也叫「提示数」），它们永远不变——整个解都挂在它们这个锚上。你要做的，是把所有空格填满。

唯一的规则

说实话只有一条，只不过重复了三遍：每一行、每一列、每一个 3×3 宫，都必须包含 1 到 9 这九个数字，且不能重复。就这样。同一行里没有哪个数字出现两次，同一列里没有，同一个宫里也没有。

因为每个单元都是九格配九个不同数字，「每个数字用一次」和「不留空」其实是同一句话。填完的盘，每一行、每一列、每一宫都恰好凑齐九个数字——不多，不少。

到底从哪儿下手

新手常常对着空格发懵。别这样。盯着已经填好的数字，反过来问：不是「这个空格该填什么」，而是「这个数字不能放在哪儿」。

挑一个在初始数字里出现得多的数——比如盘上已经有五个 7。每个 7 都禁止它所在的行、列、宫里再出现别的 7。在脑子里把这些禁区一画，你常会发现某个宫里只剩一个空格还能容下 7，那一格就必然是 7。这招——拿一个数字在全盘扫一遍——叫「划线扫描」，靠它就能啃下所有简单题。

举个例子：假设左上角那个宫还缺一个 4，而你看到它的顶行里坐着一个 4、中行里又有一个 4（都在宫外，但压在这两条线上）。这下宫里三行中有两行都不许放 4 了。如果这个宫的底行只剩一个空格，那 4 就无处可去——落定。

候选数标记

当一眼能看出的落子用光之后，就改用候选数——在格子里记下的小数字，用来追踪这里还合法的数有哪些。一个只能填 2 或 5 的格子，角上就记个小小的「2 5」。候选数能把一盘模糊的棋变成一张地图：当某格的清单缩到只剩一个数，你就找到了落子；当某个数字在一个单元里只能落在一个位置，也是同理。那些有名字的技巧，说穿了多半就是划掉候选数的各种聪明办法。

在本站，你可以在笔记模式里自己填候选数，而提示按钮永远会告诉你某个候选数为什么能被划掉，而不是替你一删了事。

新手常犯的错

瞎猜。一道规范的数独从不需要猜。一旦你想动手猜，几乎总是漏掉了某一步逻辑——慢下来，再扫一遍。
忘了宫。大家盯着行和列，却把 3×3 的宫忽略了。偏偏常常是宫的约束，才把一格撬开。
不更新候选数。每落一个数，它就该从同行、同列、同宫的候选里消失。过期的标记只会推出过期的结论。

真的，就这些

一条规则，三类单元，零算术。其余的一切——那些优雅的技巧、过瘾的推理链——都从这一条约束里长出来。想动手试试时，去开一局；等简单题不再是挑战，下一站就是解题技巧那篇。