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# Introduction to Partial K-Trees

• Last Updated : 09 Mar, 2023

In graph theory, a partial k-tree (also known as a partial k-tree decomposition) is a graph-theoretic model that can be used to represent and solve a variety of NP-hard problems. Partial k-trees are a powerful tool for tackling complex problems in computer science, as they offer a way to reduce the complexity of a graph while still preserving its essential structure. This makes them particularly useful for solving NP-hard problems, which are notoriously difficult to solve in polynomial time.

## What does Partial k-trees means?

Partial K-trees, or k-ary trees, are a type of directed acyclic graph (DAG) that serves as an efficient way to store and retrieve data from a large, sparsely connected graph. They are similar to regular K-trees, but instead of having a fixed number of nodes and edges, partial K-trees have a variable number of nodes and edges.

This makes them more flexible and efficient when dealing with large data sets that contain many sparsely connected nodes.

K-trees have a few specific properties that make them useful for certain types of problems.

• k-trees are “partially ordered”, meaning that there is a partial ordering between the nodes. This means that the order of the nodes does not matter and that any two nodes can be connected by at least one path.
• k-trees are “locally connected”, meaning that each node has at least one edge connecting it to another node. This property makes k-trees much more efficient than other types of graphs that are not locally connected.

Partial k-trees are particularly useful for problems in which the graph structure is important, as the restrictions on the number of neighbors each vertex can have, ensures that the graph is not overly cluttered or complex. This makes it easier to identify and isolate important components of the graph, which can then be used to solve the underlying problem.

## How is a partial k-Tree used to solve NP-Hard problems?

The number of edges in a partial k-tree is bounded by a polynomial of degree k. This property is known as bounded degree. If an NP-hard problem can be transformed into a subgraph of the partial k-tree, then the problem can be solved in polynomial time using algorithms designed to take advantage of the bounded degree property of the graph. This is because partial k-trees are highly structured and can be decomposed into smaller subgraphs that can be solved more efficiently

Example:

One example of an NP-hard problem that can be solved using partial k-trees is the Minimum Vertex Cover Problem

The goal of this problem is to find the smallest set of vertices that can cover all the edges in a graph. This problem is NP-hard, but can be solved by transforming it into a subgraph of a partial k-tree.

## Benefits of Partial k-Tree:

Partial k-trees offer a number of advantages over other graph models.

• They are more compact than other models, as they utilize less space in order to represent the same amount of information. This makes them ideal for large-scale applications, as they can be used to represent and manipulate graphs with millions of vertices or edges.
• Partial k-trees are versatile and can be used to represent both directed and undirected graphs. As such, they can be used to solve a variety of problems, from shortest-path problems to network flow problems.
• Partial k-trees are highly efficient, as they allow for polynomial-time algorithms to be used to solve NP-hard problems. This allows for faster solutions, as the time required to solve a given problem is greatly reduced.

## Implementation of partial k-Tree:

The implementation of a partial K-tree in C++ would require

• The use of a graph data structure such as an adjacency list or an adjacency matrix.
• The data structure would need to store the nodes and edges of the graph and be able to retrieve them efficiently.
• Additionally, the data structure would need to be able to add and remove nodes and edges in an efficient manner.

Following is an implementation of the partial k-tree.

## C++

 `// C++ implementation of partial k-tree`   `#include ` `using` `namespace` `std;`   `// A node of the partial k-tree` `class` `Node {` `public``:` `    ``int` `data;` `    ``vector children;`   `    ``Node(``int` `data) { ``this``->data = data; }` `};`   `// Function to print the partial k-tree` `void` `printPartialKTree(Node* root)` `{` `    ``// Print the data of the node` `    ``cout << root->data << ``" "``;`   `    ``// Print the children` `    ``for` `(``int` `i = 0; i < root->children.size(); i++)` `        ``printPartialKTree(root->children[i]);` `}`   `// Function to construct a partial k-tree` `Node* createPartialKTree(``int` `k)` `{` `    ``Node* root = ``new` `Node(1);`   `    ``// Create k children` `    ``for` `(``int` `i = 0; i < k; i++)` `        ``root->children.push_back(``new` `Node(i + 2));`   `    ``// Create k children for each of the k children` `    ``for` `(``int` `i = 0; i < k; i++) {` `        ``Node* temp = root->children[i];` `        ``for` `(``int` `j = 0; j < k; j++)` `            ``temp->children.push_back(` `                ``new` `Node(k * (i + 1) + j + 2));` `    ``}`   `    ``return` `root;` `}`   `// Driver code` `int` `main()` `{` `    ``int` `k = 3;`   `    ``// Function call to create the tree` `    ``Node* root = createPartialKTree(k);`   `    ``cout << ``"Partial k-tree created:\n"``;`   `    ``// Function call to print the tree` `    ``printPartialKTree(root);`   `    ``return` `0;` `}`

## Java

 `import` `java.util.ArrayList;` `import` `java.util.List;`   `// A node of the partial k-tree` `class` `Node {` `    ``public` `int` `data;` `    ``public` `List children;`   `    ``Node(``int` `data)` `    ``{` `        ``this``.data = data;` `        ``children = ``new` `ArrayList();` `    ``}` `}` `class` `GFG {` `    ``// Function to print the partial k-tree` `    ``public` `static` `void` `printPartialKTree(Node root)` `    ``{` `        ``// Print the data of the node` `        ``System.out.print(root.data + ``" "``);`   `        ``// Print the children` `        ``for` `(``int` `i = ``0``; i < root.children.size(); i++)` `            ``printPartialKTree(root.children.get(i));` `    ``}`   `    ``// Function to construct a partial k-tree` `    ``public` `static` `Node createPartialKTree(``int` `k)` `    ``{` `        ``Node root = ``new` `Node(``1``);`   `        ``// Create k children` `        ``for` `(``int` `i = ``0``; i < k; i++)` `            ``root.children.add(``new` `Node(i + ``2``));`   `        ``// Create k children for each of the k children` `        ``for` `(``int` `i = ``0``; i < k; i++) {` `            ``Node temp = root.children.get(i);` `            ``for` `(``int` `j = ``0``; j < k; j++)` `                ``temp.children.add(` `                    ``new` `Node(k * (i + ``1``) + j + ``2``));` `        ``}`   `        ``return` `root;` `    ``}`   `    ``// Driver code` `    ``public` `static` `void` `main(String[] args)` `    ``{` `        ``int` `k = ``3``;`   `        ``// Function call to create the tree` `        ``Node root = createPartialKTree(k);`   `        ``System.out.println(``"Partial k-tree created:"``);`   `        ``// Function call to print the tree` `        ``printPartialKTree(root);` `    ``}` `}`

## Python3

 `# Python implementation of the partial k-tree`   `# A node of the partial k-tree` `class` `Node:` `    ``def` `__init__(``self``, data):` `        ``self``.data ``=` `data` `        ``self``.children ``=` `[]`   `# Function to print the partial k-tree` `def` `printPartialKTree(root):` `      ``# print the data of the node` `    ``print``(root.data, end``=``" "``)` `    ``# print the children` `    ``for` `child ``in` `root.children:` `        ``printPartialKTree(child)`   `# Function to construct a partial k-tree` `def` `createPartialKTree(k):` `    ``root ``=` `Node(``1``)` `    ``# create k children` `    ``for` `i ``in` `range``(k):` `        ``root.children.append(Node(i ``+` `2``))` `    `  `    ``# Create k children for each of the k children` `    ``for` `i ``in` `range``(k):` `        ``temp ``=` `root.children[i]` `        ``for` `j ``in` `range``(k):` `            ``temp.children.append(Node(k ``*` `(i ``+` `1``) ``+` `j ``+` `2``))` `    ``return` `root`   `if` `__name__ ``=``=` `'__main__'``:` `    ``k ``=` `3` `    ``# Function call to create the tree` `    ``root ``=` `createPartialKTree(k)` `    ``print``(``"Partial k-tree created:"``)` `    ``# Function call to print the tree` `    ``printPartialKTree(root)`   `# This code is contributed by karthik.`

## C#

 `using` `System;` `using` `System.Collections.Generic;`   `// A node of the partial k-tree` `class` `Node {` `  ``public` `int` `Data;` `  ``public` `List Children;`   `  ``public` `Node(``int` `data)` `  ``{` `    ``Data = data;` `    ``Children = ``new` `List();` `  ``}` `}`   `public` `class` `GFG {`   `  ``// Function to print the partial k-tree` `  ``static` `void` `PrintPartialKTree(Node root)` `  ``{` `    ``// Print the data of the node` `    ``Console.Write(root.Data + ``" "``);`   `    ``// Print the children` `    ``foreach``(``var` `child ``in` `root.Children)` `      ``PrintPartialKTree(child);` `  ``}`   `  ``// Function to construct a partial k-tree` `  ``static` `Node CreatePartialKTree(``int` `k)` `  ``{` `    ``Node root = ``new` `Node(1);`   `    ``// Create k children` `    ``for` `(``int` `i = 0; i < k; i++)` `      ``root.Children.Add(``new` `Node(i + 2));`   `    ``// Create k children for each of the k children` `    ``for` `(``int` `i = 0; i < k; i++) {` `      ``Node temp = root.Children[i];` `      ``for` `(``int` `j = 0; j < k; j++)` `        ``temp.Children.Add(` `        ``new` `Node(k * (i + 1) + j + 2));` `    ``}`   `    ``return` `root;` `  ``}`   `  ``static` `public` `void` `Main()` `  ``{`   `    ``// Code` `    ``int` `k = 3;`   `    ``// Function call to create the tree` `    ``Node root = CreatePartialKTree(k);`   `    ``Console.WriteLine(``"Partial k-tree created:"``);`   `    ``// Function call to print the tree` `    ``PrintPartialKTree(root);` `  ``}` `}`   `// This code is contributed by lokeshmvs21.`

## Javascript

 `// JavaScript implementation of partial k-tree` `class Node {` `    ``constructor(data) {` `        ``this``.data = data;` `        ``this``.children = [];` `    ``}` `}`   `function` `printPartialKTree(root) {` `    ``// Print the data of the node` `    ``console.log(root.data + ``" "``);` `    `  `    ``// Print the children` `    ``for` `(let i = 0; i < root.children.length; i++) {` `        ``printPartialKTree(root.children[i]);` `    ``}` `}`   `function` `createPartialKTree(k) {` `    ``const root = ``new` `Node(1);` `    `  `    ``// Create k children` `    ``for` `(let i = 0; i < k; i++) {` `        ``root.children.push(``new` `Node(i + 2));` `    ``}` `    `  `    ``// Create k children for each of the k children` `    ``for` `(let i = 0; i < k; i++) {` `        ``const temp = root.children[i];` `        ``for` `(let j = 0; j < k; j++) {` `            ``temp.children.push(``new` `Node(k * (i + 1) + j + 2));` `        ``}` `    ``}`   `    ``return` `root;` `}`   `// Driver code` `const k = 3;`   `// Function call to create the tree` `const root = createPartialKTree(k);`   `console.log(``"Partial k-tree created:
"``);`   `// Function call to print the tree` `printPartialKTree(root);`   `// This code is contributed by sankar.`

Output

```Partial k-tree created:
1 2 5 6 7 3 8 9 10 4 11 12 13 ```

### Complexity Analysis:

Time Complexity: O(k2) because we need to create k children for each of the k children which takes O(k^2) time.
Auxiliary Space: O(k2).
As we need to create k^2 nodes for the partial k-tree.

## Conclusion

Partial k-trees are a powerful graph model that can be used to represent and solve a variety of NP-hard problems in polynomial time. By restricting the graph structure to a partial k-tree, it is possible to reduce the complexity of a problem while still preserving its essential structure. This makes partial k-trees a great tool for tackling complex problems in computer science, as they offer a way to efficiently analyze and manipulate graphs.

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