JavaScript Data Structures - Tree

Definition

A tree is a data structure consisting of a set of linked nodes that represent a hierarchical tree structure. Each node is linked to others via parent-children relationship. The first node in the tree is the root, whereas nodes without any children are the leaves.

Each node in a tree data structure must have the following properties:

• key: The key of the node
• value: The value of the node
• parent: The parent of the node (null if there is none)
• children: An array of pointers to the node's children

The main operations of a tree data structure are:

• insert: Inserts a node as a child of the given parent node
• remove: Removes a node and its children from the tree
• find: Retrieves a given node
• preOrderTraversal: Traverses the tree by recursively traversing each node followed by its children
• postOrderTraversal: Traverses the tree by recursively traversing each node's children followed by the node

Implementation

class TreeNode {
  constructor(key, value = key, parent = null) {
    this.key = key;
    this.value = value;
    this.parent = parent;
    this.children = [];
  }

  get isLeaf() {
    return this.children.length === 0;
  }

  get hasChildren() {
    return !this.isLeaf;
  }
}

class Tree {
  constructor(key, value = key) {
    this.root = new TreeNode(key, value);
  }

  *preOrderTraversal(node = this.root) {
    yield node;
    if (node.children.length) {
      for (let child of node.children) {
        yield* this.preOrderTraversal(child);
      }
    }
  }

  *postOrderTraversal(node = this.root) {
    if (node.children.length) {
      for (let child of node.children) {
        yield* this.postOrderTraversal(child);
      }
    }
    yield node;
  }

  insert(parentNodeKey, key, value = key) {
    for (let node of this.preOrderTraversal()) {
      if (node.key === parentNodeKey) {
        node.children.push(new TreeNode(key, value, node));
        return true;
      }
    }
    return false;
  }

  remove(key) {
    for (let node of this.preOrderTraversal()) {
      const filtered = node.children.filter(c => c.key !== key);
      if (filtered.length !== node.children.length) {
        node.children = filtered;
        return true;
      }
    }
    return false;
  }

  find(key) {
    for (let node of this.preOrderTraversal()) {
      if (node.key === key) return node;
    }
    return undefined;
  }
}

Undirected tree from edges

Learn how to build an undirected tree from an array of edges in JavaScript, with various approaches for storing node relationships.

• Create a class for the TreeNode with a constructor that initializes the appropriate key, value, parent and children properties.
• Define an isLeaf getter, that uses Array.prototype.length to check if children is empty.
• Define a hasChildren getter, that is the reverse of the isLeaf getter.
• Create a class for the Tree with a constructor that initializes the root of the tree.
• Define a preOrderTraversal() generator method that traverses the tree in pre-order, using the yield* syntax to recursively delegate traversal to itself.
• Define a postOrderTraversal() generator method that traverses the tree in post-order, using the yield* syntax to recursively delegate traversal to itself.
• Define an insert() method, that uses the preOrderTraversal() method and Array.prototype.push() to add a new TreeNode to the tree.
• Define a remove() method, that uses the preOrderTraversal() method and Array.prototype.filter() to remove a TreeNode from the tree.
• Define a find() method, that uses the preOrderTraversal() method to retrieve the given node in the tree.

const tree = new Tree(1, 'AB');

tree.insert(1, 11, 'AC');
tree.insert(1, 12, 'BC');
tree.insert(12, 121, 'BG');

[...tree.preOrderTraversal()].map(x => x.value);
// ['AB', 'AC', 'BC', 'BCG']

tree.root.value;              // 'AB'
tree.root.hasChildren;        // true

tree.find(12).isLeaf;         // false
tree.find(121).isLeaf;        // true
tree.find(121).parent.value;  // 'BC'

tree.remove(12);

[...tree.postOrderTraversal()].map(x => x.value);
// ['AC', 'AB']