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

# Combinatorial analysis and techniques

The Mathematical Background

## Permutations and Combinations

Permutations and combinations are important techniques in combinatorial analysis that are used in programming.

### Permutations

A permutation is an arrangement of objects in a specific order. The number of permutations of n distinct objects taken r at a time is given by n!/(n-r)!. The symbol n! (read "n factorial") represents the product of all positive integers up to n. For example, 5! = 5 x 4 x 3 x 2 x 1 = 120.

Permutations are used in many programming problems, such as finding all possible arrangements of a set of characters or finding all possible orders in which a group of tasks can be performed.

### Combinations

A combination is a selection of objects from a set, where the order of selection does not matter. The number of combinations of n distinct objects taken r at a time is given by n!/(r!(n-r)!).

Combinations are used in programming problems where the order of selection is not important, such as selecting a team of players from a pool of candidates or selecting a subset of items from a larger set.

Understanding permutations and combinations is essential for many programming problems, and they are often used in conjunction with other combinatorial techniques.

## Generating Functions

Generating functions are a powerful tool for representing combinatorial sequences. A generating function is a formal power series whose coefficients represent the terms of a combinatorial sequence. By manipulating the generating function algebraically, we can extract information about the sequence, such as its generating function, its closed-form expression, and its generating function's asymptotic behavior.

Generating functions have many applications in combinatorial analysis and algorithmic analysis. For example, they can be used to solve recurrences, count the number of partitions of a set, and enumerate the number of ways to arrange a set of objects subject to certain constraints.

There are many different types of generating functions, including ordinary generating functions, exponential generating functions, and Dirichlet generating functions. Each type of generating function is suited to different types of combinatorial sequences and problems.

## 3. Graph Algorithms

Graph algorithms are fundamental to computer programming. A graph is a collection of vertices (or nodes) and edges that connect them. Graph algorithms are used to search for paths between vertices, find connected components, and determine whether a graph is acyclic or bipartite.

### Types of Graph Algorithms

There are many types of graph algorithms, including:

- Depth-First Search (DFS): This algorithm traverses a graph by exploring as far as possible along each branch before backtracking.
- Breadth-First Search (BFS): This algorithm traverses a graph by exploring all the neighbor vertices at the current depth before moving on to the vertices at the next depth level.
- Dijkstra's Algorithm: This algorithm finds the shortest path between two vertices in a weighted graph.
- Bellman-Ford Algorithm: This algorithm finds the shortest path in a weighted graph with negative edges.
- Floyd-Warshall Algorithm: This algorithm finds the shortest path between all pairs of vertices in a weighted graph.
- Kruskal's Algorithm: This algorithm finds the minimum spanning tree of a weighted graph.
- Prim's Algorithm: This algorithm finds the minimum spanning tree of a weighted graph.

### Applications of Graph Algorithms

Graph algorithms have many applications in computer programming, including:

- Network routing
- Social network analysis
- Web crawling
- Image processing
- Game AI

## 4. Dynamic Programming

Dynamic programming is a method for solving complex problems by breaking them down into smaller, simpler subproblems. The key idea behind dynamic programming is to store the solutions to subproblems, so that they can be reused later on. This can lead to significant performance improvements, as many subproblems are often repeated in the solution to a larger problem.

Dynamic programming can be used to solve a wide range of problems, including optimization problems, counting problems, and decision problems. It is especially useful when the problem can be broken down into a set of overlapping subproblems.

The basic steps involved in using dynamic programming to solve a problem are:

- Define the subproblems: Break the problem down into a set of smaller subproblems.
- Define the base cases: Identify the simplest subproblems that can be solved directly.
- Define the recursive relation: Define how the solution to a larger problem can be obtained from the solutions to its subproblems.
- Implement the algorithm: Use the recursive relation and the base cases to solve the problem.

Dynamic programming algorithms can be classified into two types: top-down and bottom-up. In top-down dynamic programming, the problem is solved recursively, starting from the largest subproblem and working down to the base cases. In bottom-up dynamic programming, the problem is solved iteratively, starting from the base cases and working up to the largest subproblem.

Dynamic programming has many applications in computer programming, including:

- Knapsack problem: Given a set of items with weights and values, determine the maximum value that can be obtained by selecting a subset of the items that fit into a knapsack of a given capacity.
- Longest common subsequence: Given two sequences, find the longest subsequence that is common to both sequences.
- Edit distance: Given two sequences, find the minimum number of operations required to transform one sequence into the other.
- Shortest path: Given a graph with weighted edges, find the shortest path between two vertices.
- Maximum flow: Given a network with capacities on the edges, find the maximum flow that can be sent from a source vertex to a sink vertex.

## 5. Backtracking

Backtracking is a technique for exhaustive searching of all possible solutions to a problem. The technique is based on the idea of constructing a solution incrementally and abandoning a candidate solution as soon as it is determined to be infeasible. By exploring all possible solutions systematically, backtracking can find the optimal solution to a problem.

### Applications of Backtracking

Backtracking is used in many programming problems, such as:

- N-Queens Problem: Given an NxN chessboard, place N queens on the board such that no two queens attack each other.
- Subset Sum Problem: Given a set of integers and a target value, find a subset of the integers that adds up to the target value.
- Sudoku: Given an incomplete Sudoku board, fill in the missing numbers such that each row, column, and 3x3 subgrid contains the numbers 1 to 9.
- Hamiltonian Path: Given a graph, find a path that visits every vertex exactly once.
- Traveling Salesman Problem: Given a set of cities and the distances between them, find the shortest possible route that visits each city exactly once and returns to the starting city.

### Backtracking Algorithm

The basic steps involved in using backtracking to solve a problem are:

- Define the problem: Define the problem and the constraints that must be satisfied.
- Define the candidates: Determine the set of candidates that can be used to construct a solution.
- Define the solution vector: Define the data structure that will be used to store the solution.
- Define the feasibility function: Define a function that checks whether a partial solution is feasible.
- Define the objective function: Define a function that evaluates the quality of a solution.
- Define the search function: Define a recursive function that constructs a solution incrementally and abandons candidates as soon as they are determined to be infeasible.
- Implement the algorithm: Use the search function to find the optimal solution to the problem.

Backtracking is a powerful technique for solving many types of programming problems. However, it can be computationally expensive, especially for large problems. It is important to carefully choose the candidates and the feasibility function to minimize the search space and improve performance.

## 6. Greedy Algorithms

Greedy algorithms are a class of algorithms that make locally optimal choices at each step, with the hope of finding a global optimal solution. The key idea behind greedy algorithms is to make the best possible decision at each step, without worrying about the long-term consequences.

### How Greedy Algorithms Work

Greedy algorithms work by maintaining a set of candidates that can be used to construct a solution. At each step, the algorithm selects the candidate that appears to be the best choice at that moment, based on some predefined criteria. The selected candidate is added to the solution, and the process is repeated until a complete solution is obtained.

### Examples of Greedy Algorithms

There are many examples of greedy algorithms, including:

- Huffman coding: This is a compression algorithm that assigns shorter codes to more frequently occurring symbols.
- Dijkstra's algorithm for the shortest path problem: This algorithm finds the shortest path between two vertices in a weighted graph by making locally optimal choices at each step.
- Kruskal's algorithm for the minimum spanning tree problem: This algorithm finds the minimum spanning tree of a weighted graph by adding edges in increasing order of their weights, as long as they do not form a cycle.
- Activity selection problem: Given a set of activities with start and end times, find the maximum number of activities that can be scheduled without overlapping.
- Fractional knapsack problem: Given a set of items with weights and values, determine the maximum value that can be obtained by selecting a subset of the items that fit into a knapsack of a given capacity, where fractions of items can be taken.

### Advantages and Disadvantages of Greedy Algorithms

The advantages of greedy algorithms are that they are simple to implement, efficient, and can often find good solutions quickly. However, greedy algorithms do not always find the optimal solution, and their performance can depend heavily on the criteria used to make decisions at each step.

### When to Use Greedy Algorithms

Greedy algorithms are best suited to problems where the optimal solution can be constructed by making locally optimal choices at each step. They are not suitable for problems where the optimal solution depends on the entire input or where the selection of one candidate affects the feasibility of other candidates.