## The Mastery of Computer Programming: Primary Algorithms - Sykalo Eugene 2023

# Dancing Links

Algorithms for Combinatorial Problems

## Background

Combinatorial problems deal with the study of finite or countable discrete structures. These problems can be broken down into smaller sub-problems, which can then be solved using a systematic approach. Fundamental algorithms provide a general framework for solving complex combinatorial problems. By using these algorithms, it is possible to solve a wide range of problems in an efficient and scalable manner. The use of fundamental algorithms is particularly important in fields such as computer science, mathematics, and operations research.

## Dancing Links Algorithm

The Dancing Links algorithm is a technique for implementing backtracking algorithms for exact cover problems. The algorithm was developed by Donald Knuth in his book, "Dancing Links". The algorithm is based on the idea of representing the exact cover problem as a matrix, where each row represents a constraint that must be satisfied and each column represents a set of elements that can satisfy the constraints. The algorithm uses a circular doubly linked list to represent the matrix. The links in the list allow for efficient backtracking and pruning of search space. The algorithm starts by selecting a column with the smallest number of ones and then iteratively selects rows that contain a one in that column. If a row is selected, all columns that contain a one in that row are removed from the matrix, along with all rows that contain a one in those columns. The algorithm terminates when the matrix is empty or when it is not possible to select a row for a given column. The algorithm's efficiency and versatility make it a valuable tool for solving a wide range of problems, including Sudoku, N-Queens, and graph coloring problems.

## Implementation

The Dancing Links algorithm involves representing the exact cover problem as a matrix, where each row represents a constraint that must be satisfied and each column represents a set of elements that can satisfy the constraints. The algorithm uses a circular doubly linked list to represent the matrix. Each node in the list represents a one in the matrix. The nodes are connected horizontally and vertically, forming a circular doubly linked list. The links in the list allow for efficient backtracking and pruning of search space.

The algorithm starts by selecting a column with the smallest number of ones. This heuristic helps to reduce the size of the search space. After selecting a column, the algorithm iteratively selects rows that contain a one in that column. If a row is selected, all columns that contain a one in that row are removed from the matrix, along with all rows that contain a one in those columns. This process is repeated until the matrix is empty or until it is not possible to select a row for a given column.

The implementation of the Dancing Links algorithm is efficient and compact. The use of the circular doubly linked list allows for fast backtracking and the ability to restore the matrix to its original state after each iteration. The algorithm can handle large search spaces and has been used to solve a variety of combinatorial problems, including Sudoku, N-Queens, and graph coloring problems.

The Dancing Links algorithm has found numerous applications in various fields due to its efficiency and versatility. Some of the notable applications of the algorithm are discussed below:

- Sudoku: Sudoku is a popular puzzle game that involves filling a 9x9 grid with digits such that each row, column, and 3x3 subgrid contains all the digits from 1 to 9. The Dancing Links algorithm can be used to solve Sudoku puzzles by representing the puzzle as an exact cover problem. The algorithm can quickly find the solution by efficiently exploring the search space.
- N-Queens: The N-Queens problem involves placing N chess queens on an NxN chessboard such that no two queens attack each other. The Dancing Links algorithm can be used to solve the problem by representing it as an exact cover problem. The algorithm can efficiently explore the search space and find the solution.
- Graph coloring: Graph coloring is the problem of assigning colors to the vertices of a graph such that no two adjacent vertices have the same color. The Dancing Links algorithm can be used to solve graph coloring problems by representing them as exact cover problems. The algorithm can explore the search space and find the solution efficiently.
- Tiling puzzles: Tiling puzzles involve covering a rectangular area with a set of tiles of different shapes and sizes. The Dancing Links algorithm can be used to solve tiling puzzles by representing them as exact cover problems. The algorithm can explore the search space and find the solution efficiently.
- Scheduling problems: Scheduling problems involve assigning tasks to resources subject to various constraints. The Dancing Links algorithm can be used to solve scheduling problems by representing them as exact cover problems. The algorithm can explore the search space and find the optimal solution.