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The most di

cult Sudoku puzzles are quickly solved

by a straightforward depth- rst search algorithm
Armando B. Matos
[email protected]
LIACC
Arti cial Intelligence and Computer Science Laboratory
Universidade do Porto

September 9, 2016

Contents
1 Introduction
2 A brief note on the technique of \generate and test"

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3 \Straightforward" depth- rst search (SDFS) technique

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3.1 General comments . . . . . . . . . . . . . . . . . . . . . . . . . .

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3.2 Visits and branching . . . . . . . . . . . . . . . . . . . . . . . . .

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3.3 A few examples analysed in detail . . . . . . . . . . . . . . . . .

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3.3.1 Example A: a \very di cult" puzzle, 24 clues . . . . . . .

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3.3.2 Example B: a \di cult" minimum puzzle . . . . . . . . .

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3.3.3 Example B with the order of the rows reversed . . . . . .

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3.3.4 Example C: from [3], minimum puzzle . . . . . . . . . . .

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3.3.5 Example D: a minimum puzzle from [2] . . . . . . . . . .

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3.3.6 Example E: \only for the sharpest