先掌握三個限制
每行、每列與每個九宮格都只能出現一次 1 到 9。任何候選數都必須同時滿足三個限制。
唯一候選
若空格排除後只剩一個可能數字，就可以直接填入。新手應先掃描缺一個數字的行列。
唯一位置
有時某數字在一個九宮格中只剩一個可放位置，即使該格仍有其他候選，也能確定該數字。
交叉排除
九宮格中某數字若只能落在同一行，該行其他九宮格就不能再放此數字。這是從基礎進入中階的重要技巧。
不要猜
猜測會把錯誤延後到很久之後。卡住時重新檢查候選、唯一位置和交叉限制。
掃描的順序比技巧更重要
新手常常隨機找空格，效率很低。建議固定順序：先按數字掃（從 1 到 9，每個數字找它在各宮還能放哪裡），再按宮掃（哪一宮只剩兩三個空格），最後才按行列掃。同一套順序走三輪之後，你會發現多數簡單題根本不需要記候選數。
什麼時候該開始寫候選數
當連續兩輪掃描都填不進任何一格時，就是該寫候選數的時候。寫的時候只寫還剩三個以內候選的格子——把全盤都寫滿反而會製造視覺噪音。候選數寫下來之後，優先找「數對」：同一行或同一宮裡有兩格的候選完全相同且只有兩個數，那這兩個數就能從該行/宮的其他格中刪掉。
常見的三個卡關原因
- 漏看九宮格限制：只盯著行與列，忘了第三個限制，這是初學者最常見的盲點。
- 候選數沒有及時更新：填進一個數字之後，要立刻刪掉同行、同列、同宮裡對應的候選，否則後面全是錯的。
- 用猜的：數獨每一題都有唯一解且可純邏輯推出。猜錯不會立刻報錯，而是在十幾步之後才爆炸，屆時已經很難回溯。
Three constraints
Each row, column and 3×3 box contains the digits 1–9 once. Every candidate must satisfy all three constraints.
Naked singles
If elimination leaves one possible number in a cell, place it. Beginners should first scan rows and columns missing only one value.
Hidden singles
A number may have only one possible position in a box even when that cell has other candidates. That position is still forced.
Box-line elimination
If a number in a box is restricted to one row, the same number can be removed from that row in other boxes. This is a key intermediate technique.
Avoid guessing
Guessing delays an error. When stuck, rescan candidates, hidden singles and cross-box restrictions.
The scanning order matters more than the techniques
Beginners often hunt for empty cells at random, which is slow. Use a fixed order instead: scan by digit first (take 1 through 9 and ask where each can still go in every box), then by box (which box has only two or three empties left), and only then by row and column. After three passes in that order, most easy puzzles turn out not to need pencil marks at all.
When to start writing candidates
The moment two consecutive scans place nothing, it is time for pencil marks. Write them only in cells that are down to three or fewer candidates — filling in the whole grid creates visual noise instead of clarity. Once the marks are down, look first for pairs: when two cells in the same row or box carry exactly the same two candidates, those two digits can be removed from every other cell in that unit.
Three common reasons for getting stuck
- Forgetting the box constraint. Watching only rows and columns is the most common beginner blind spot.
- Stale candidates. After placing a digit, immediately remove it from the candidates in that row, column and box, or everything after it is built on wrong information.
- Guessing. Every valid sudoku has one solution reachable by logic alone. A wrong guess does not fail immediately — it detonates a dozen moves later, when backtracking is hardest.