Knuth's Algorithm X: Difference between revisions

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{{Short description|Algorithm for exact cover problem}}

'''Algorithm X''' is an [[algorithm]] for solving the [[exact cover]] problem. It is a straightforward [[Recursion (computer science)|recursive]], [[Nondeterministic algorithm|nondeterministic]], [[depth-first]], [[backtracking]] algorithm used by [[Donald Knuth]] to demonstrate an efficient implementation called DLX, which uses the [[dancing links]] technique.<ref name="knuth">{{cite arXiv | author = Knuth, Donald | author-link = Donald Knuth | title = Dancing links | year = 2000 | eprint = cs/0011047 }}</ref><ref>{{Cite journal |last=Banerjee |first=Bikramjit |last2=Kraemer |first2=Landon |last3=Lyle |first3=Jeremy |date=2010-07-04 |title=Multi-Agent Plan Recognition: Formalization and Algorithms |url=https://ojs.aaai.org/index.php/AAAI/article/view/7746 |journal=Proceedings of the AAAI Conference on Artificial Intelligence |volume=24 |issue=1 |pages=1059–1064 |doi=10.1609/aaai.v24i1.7746 |issn=2374-3468|doi-access=free }}</ref>

==Algorithm==

The exact cover problem is represented in Algorithm X by aan [[incidence matrix]] ''A'' consisting of 0s and 1s. The goal is to select a subset of the rows such that the digit 1 appears in each column exactly once.

Algorithm X works as follows:

{{quote frame|quote=

# If the matrix ''A'' has no columns, the current partial solution is a valid solution; terminate successfully.

# Otherwise choose a column ''c'' ([[deterministic algorithm|deterministically]]).

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#: delete column ''j'' from matrix ''A''.

# Repeat this algorithm recursively on the reduced matrix ''A''.

}}

The nondeterministic choice of ''r'' means that the algorithm recurses over independent subalgorithms; each subalgorithm inherits the current matrix ''A'', but reduces it with respect to a different row ''r''.

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:: Step 1—The matrix is not empty, so the algorithm proceeds.

:: Step 2—The lowest number of 1s in any column is one. Column 2 is the first column with one 1 and thus is selected (deterministically).:

:::{| class="wikitable"

! !! <span style="color:red">2</span> !! 7

|-

! ''F''

| <span style="color:red;font-weight:bold">1</span> || 1

}|}

:: Row ''F'' has a 1 in column 2 and thus is selected (nondeterministically).

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! <span style="color:blue">''F''</span>

| <span style="color:red;font-weight:bold">1</span> || <span style="color:red;font-weight:bold">1</span>

|}

::: No rows and no columns remain:

::::{| class="wikitable"

! &nbsp;

|}

::: Step 1—The matrix is empty, thus this branch of the algorithm terminates successfully.

::: As rows ''B'', ''D'', and ''F'' arehave been selected (step 4), the final solution in this branch is:

::::{| class="wikitable"

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==Implementations==

[[Donald Knuth]]'s main purpose in describing Algorithm X was to demonstrate the utility of [[dancing links]]. Knuth showed that Algorithm X can be implemented efficiently on a computer using dancing links in a process Knuth calls ''"DLX"''. DLX uses the matrix representation of the [[exact cover]] problem, implemented as [[doubly linked list]]s of the 1s of the matrix: each 1 element has a link to the next 1 above, below, to the left, and to the right of itself. (Technically, because the lists are circular, this forms a [[torus]]). Because exact cover problems tend to be sparse, this representation is usually much more efficient in both size and processing time required. DLX then uses dancing links to quickly select permutations of rows as possible solutions and to efficiently backtrack (undo) mistaken guesses.<ref name="knuth" />

==See also==