Algorithm X is an algorithm for solving the exact cover problem. It is a straightforward recursive, nondeterministic, depth-first, backtracking algorithm used by Donald Knuth to demonstrate an efficient implementation called DLX, which uses the dancing links technique. The exact cover problem is represented in Algorithm X by a 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:
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| - Algorithme X de Knuth (fr)
- Knuth's Algorithm X (en)
- X算法 (zh)
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| - L'algorithme X de Donald Knuth est un algorithme récursif (en), de parcours en profondeur et à retour sur trace. Il permet de trouver des solutions au problème de la couverture exacte, représenté sous la forme d'une matrice contenant des 0 et des 1. L'objectif est de déterminer un sous-ensemble de lignes tel que le chiffre 1 n'apparaisse dans chaque colonne qu'une et une seule fois. (fr)
- 在计算机科学中，X算法可用来求解精确覆盖问题。此名称最早在高德纳的论文《舞蹈链》中出现，他认为此算法是“试错法中最显而易见”的。 就技术而言，X算法是一个深度优先的回溯算法。由于X算法是一个解决精确覆盖问题的简洁方法，高德纳希望通过该算法体现舞蹈链数据结构的高效性，他把使用后者的X算法称为DLX。 X算法用由0和1组成的矩阵A来表示精确覆盖问题，目标是选出矩阵的若干行，使得其中的1在所有列中出现且仅出现一次。 X算法的步骤如下： 选择r的不确定性意味着算法将衍生出若干独立的子算法，每个子算法都从其父算法中继承了去除部分行列的A矩阵。如果其中有一列全为零，则当前情况无解，子算法返回失败，但不一定意味整个问题无解。 实际上，所有子算法形成了一棵搜索树，其中原问题为根节点，树的第k层由子算法在第k次所选择的行组成。整个算法即用回溯法对搜索树深度优先遍历。 第二步中，无论用什么方法选择列最终都可以得到解，但有的方法效率明显较高。为减少迭代次数，高德纳建议每次都选取1最少的列。 (zh)
- Algorithm X is an algorithm for solving the exact cover problem. It is a straightforward recursive, nondeterministic, depth-first, backtracking algorithm used by Donald Knuth to demonstrate an efficient implementation called DLX, which uses the dancing links technique. The exact cover problem is represented in Algorithm X by a 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: (en)
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| - Algorithm X is an algorithm for solving the exact cover problem. It is a straightforward recursive, nondeterministic, depth-first, backtracking algorithm used by Donald Knuth to demonstrate an efficient implementation called DLX, which uses the dancing links technique. The exact cover problem is represented in Algorithm X by a 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: # If the matrix A has no columns, the current partial solution is a valid solution; terminate successfully. 1.
* Otherwise choose a column c (deterministically). 2.
* Choose a row r such that Ar, c = 1 (nondeterministically). 3.
* Include row r in the partial solution. 4.
* For each column j such that Ar, j = 1,for each row i such that Ai, j = 1,delete row i from matrix A.delete column j from matrix A. 5.
* 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.If column c is entirely zero, there are no subalgorithms and the process terminates unsuccessfully. The subalgorithms form a search tree in a natural way, with the original problem at the root and with level k containing each subalgorithm that corresponds to k chosen rows.Backtracking is the process of traversing the tree in preorder, depth first. Any systematic rule for choosing column c in this procedure will find all solutions, but some rules work much better than others.To reduce the number of iterations, Knuth suggests that the column-choosing algorithm select a column with the smallest number of 1s in it. (en)
- L'algorithme X de Donald Knuth est un algorithme récursif (en), de parcours en profondeur et à retour sur trace. Il permet de trouver des solutions au problème de la couverture exacte, représenté sous la forme d'une matrice contenant des 0 et des 1. L'objectif est de déterminer un sous-ensemble de lignes tel que le chiffre 1 n'apparaisse dans chaque colonne qu'une et une seule fois. (fr)
- 在计算机科学中，X算法可用来求解精确覆盖问题。此名称最早在高德纳的论文《舞蹈链》中出现，他认为此算法是“试错法中最显而易见”的。 就技术而言，X算法是一个深度优先的回溯算法。由于X算法是一个解决精确覆盖问题的简洁方法，高德纳希望通过该算法体现舞蹈链数据结构的高效性，他把使用后者的X算法称为DLX。 X算法用由0和1组成的矩阵A来表示精确覆盖问题，目标是选出矩阵的若干行，使得其中的1在所有列中出现且仅出现一次。 X算法的步骤如下： 选择r的不确定性意味着算法将衍生出若干独立的子算法，每个子算法都从其父算法中继承了去除部分行列的A矩阵。如果其中有一列全为零，则当前情况无解，子算法返回失败，但不一定意味整个问题无解。 实际上，所有子算法形成了一棵搜索树，其中原问题为根节点，树的第k层由子算法在第k次所选择的行组成。整个算法即用回溯法对搜索树深度优先遍历。 第二步中，无论用什么方法选择列最终都可以得到解，但有的方法效率明显较高。为减少迭代次数，高德纳建议每次都选取1最少的列。 (zh)
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