Here’s a Java code example that demonstrates how to transform a List of Nodes into a Tree structure. I’ve said before that – it is the most common data structure around the internet. In this example, each node has an identifier and a parent identifier. The algorithm iterates through the list of nodes and builds the tree accordingly:
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
class TreeNode {
String id;
List children;
TreeNode(String id) {
this.id = id;
this.children = new ArrayList<>();
}}public class TreeBuilder {public static TreeNode buildTree(List<Node> nodes) {
Map<String, TreeNode> nodeMap = new HashMap<>();
TreeNode root = null;
// Create a map of node id to TreeNode
for (Node node : nodes) {
TreeNode treeNode = new TreeNode(node.id);
nodeMap.put(node.id, treeNode);
}
// Iterate through the nodes to build the tree
for (Node node : nodes) {
TreeNode treeNode = nodeMap.get(node.id);
if (node.parentId == null) {
root = treeNode; // Root node found
} else {
TreeNode parentNode = nodeMap.get(node.parentId);
if (parentNode != null) {
parentNode.children.add(treeNode); // Add current node as child to parent node
} else {
// Handle case where parent node is not found
System.err.println("Parent node not found for node with id: " + node.id);
}
}
}
return root;
}
public static void main(String[] args) {
// Sample list of nodes
List<Node> nodes = new ArrayList<>();
nodes.add(new Node("1", null));
nodes.add(new Node("2", "1"));
nodes.add(new Node("3", "1"));
nodes.add(new Node("4", "2"));
nodes.add(new Node("5", "2"));
nodes.add(new Node("6", "3"));
TreeNode root = buildTree(nodes);
// Example: Print the tree structure
printTree(root, 0);
}
// Helper method to print the tree structure
public static void printTree(TreeNode node, int level) {
if (node == null) return;
for (int i = 0; i < level; i++) {
System.out.print("\t");
}
System.out.println(node.id);
for (TreeNode child : node.children) {
printTree(child, level + 1);
}
}}class Node {
String id;
String parentId;
Node(String id, String parentId) {
this.id = id;
this.parentId = parentId;
}
}
This code first creates a map of node identifiers to TreeNode
objects. Then, it iterates through the List of Nodes, creating Tree Node
java objects and adding them from the list to the map. Finally, it iterates through the list again, building the tree structure by adding child nodes to their respective parent nodes. The root of the tree is returned at the end.
All the above logic may be skipped – if the storage was standard object serialization, json conversion, graph database or something else.
It is required when you save a tree scructure in a relational database. Usually – one could grab up to the second level of the tree – using join. Retreiving the relations with ORM often will lead to recoursive joins with raising amount.