Hard Sudoku
Choose a number, and place it in the grid above.
Hard Sudoku: The Ultimate Battle of Logic
Hard Sudoku is designed for players who have mastered the fundamentals and are ready to push their cognitive boundaries. At this level, the initial clues are often stripped down to just 20 to 25 numbers, leaving the grid looking dauntingly empty.
Solving a Hard Sudoku is no longer a linear process of discovery. It is a deep strategic game of inference and logical chains. You are no longer just looking for where a number "is"—you are deducing where a number "cannot be."
Advanced Techniques for Hard Sudoku
When standard scanning and basic pairs fail to yield progress, you must deploy these high-level logical "weapons":
1. The X-Wing
The X-Wing is the gateway to advanced Sudoku strategy. It relies on finding a pattern across two rows and two columns.
The Logic: If a specific number (e.g., 9) appears as a candidate in only two positions in Row A, and those positions align perfectly with the only two possible positions for 9 in Row B, you have formed a rectangle.
The Result: Since the 9 must occupy the corners of this rectangle diagonally, you can safely eliminate the number 9 from every other cell in those two columns.
2. The Swordfish
Think of the Swordfish as the X-Wing's more powerful older brother, involving three rows and three columns.
The Logic: If a number appears as a candidate in only two or three spots across three different rows, and all those spots fall within the same three columns, you’ve trapped that number in a logical grid.
The Result: You can remove that number from any other cells in those three columns, often breaking open a stuck puzzle.
3. The Y-Wing (Bent Triple)
This technique uses a pivot cell to create a "pincer" effect across the board.
The Logic: Look for three cells that each contain only two candidates (bi-value cells). For example: Cell A (1,2), Cell B (1,3), and Cell C (2,3). If Cell A is the "pivot" that sees both B and C, then no matter if A is 1 or 2, either B or C must result in a 3.
The Result: Any cell that is visible to both "pincer" cells (B and C) cannot contain a 3. Removing that 3 usually triggers a chain reaction of new moves.
Habits of Elite Players
Full Candidate Marking
At this level, mental tracking is impossible. Accurately noting every possible candidate for every empty cell is the mandatory foundation for identifying advanced patterns.
Identifying Logical Chains
Hard puzzles require finding "Links"—specifically Strong Links (if one is false, the other must be true). Connecting these links allows you to build XY-Chains that can eliminate candidates across the entire grid.
Embracing the "Stall"
The joy of Hard Sudoku is the "Aha!" moment after a long stalemate. Deleting a single tiny candidate through a complex X-Wing often causes the entire puzzle to collapse into a satisfying solve.