Tree Traversal Postorder Left Right Root in DSA Typescript - Build from Scratch

Tree Traversal Postorder Left Right Root

📖 Scenario: You are working with a simple family tree where each person can have up to two children. You want to visit all family members in a special order: first the left child, then the right child, and finally the parent. This order is called postorder traversal.

🎯 Goal: Build a small tree using nodes, then write a function to visit all nodes in postorder (left, right, root) and collect their names in an array.

📋 What You'll Learn

Create a TreeNode class with name, left, and right properties

Create a tree with root node named "Grandparent", left child "Parent1", and right child "Parent2"

Add children to "Parent1": left child "Child1" and right child "Child2"

Write a function postorderTraversal that takes a TreeNode and returns an array of names in postorder

Print the array returned by postorderTraversal when called on the root node

💡 Why This Matters

🌍 Real World

Tree traversal is used in many real-world applications like file system navigation, organizing family trees, and parsing expressions.

💼 Career

Understanding tree traversal is important for software developers working with hierarchical data, databases, and algorithms.

1

Create the TreeNode class and build the tree

Create a class called TreeNode with a constructor that takes a name string and optional left and right TreeNode parameters. Then create the tree nodes exactly as follows: root node named "Grandparent", left child named "Parent1", right child named "Parent2", and add children to "Parent1" named "Child1" (left) and "Child2" (right). Connect the nodes properly.

DSA Typescript

// Define TreeNode class and create the tree nodes
// Your code here

Hint

Start by defining the TreeNode class with name, left, and right. Then create each node and connect them as children.

2

Create an array to collect traversal results

Create an empty array called result of type string[] that will store the names of nodes visited during traversal.

DSA Typescript

class TreeNode {
    name: string;
    left: TreeNode | null;
    right: TreeNode | null;

    constructor(name: string, left: TreeNode | null = null, right: TreeNode | null = null) {
        this.name = name;
        this.left = left;
        this.right = right;
    }
}

const child1 = new TreeNode("Child1");
const child2 = new TreeNode("Child2");
const parent1 = new TreeNode("Parent1", child1, child2);
const parent2 = new TreeNode("Parent2");
const grandparent = new TreeNode("Grandparent", parent1, parent2);

// Create an empty array called result to store node names
// Your code here

Hint

Just create an empty array named result to hold the names.

3

Write the postorderTraversal function

Write a function called postorderTraversal that takes a node of type TreeNode | null. If the node is not null, it should recursively call itself on the left child, then on the right child, and finally push the node.name into the result array.

DSA Typescript

class TreeNode {
    name: string;
    left: TreeNode | null;
    right: TreeNode | null;

    constructor(name: string, left: TreeNode | null = null, right: TreeNode | null = null) {
        this.name = name;
        this.left = left;
        this.right = right;
    }
}

const child1 = new TreeNode("Child1");
const child2 = new TreeNode("Child2");
const parent1 = new TreeNode("Parent1", child1, child2);
const parent2 = new TreeNode("Parent2");
const grandparent = new TreeNode("Grandparent", parent1, parent2);

const result: string[] = [];

// Write the postorderTraversal function below
// Your code here

Hint

Use recursion: call postorderTraversal on left, then right, then add current node's name to result.

4

Call the function and print the result

Call postorderTraversal with the grandparent node. Then print the result array using console.log(result).

DSA Typescript

class TreeNode {
    name: string;
    left: TreeNode | null;
    right: TreeNode | null;

    constructor(name: string, left: TreeNode | null = null, right: TreeNode | null = null) {
        this.name = name;
        this.left = left;
        this.right = right;
    }
}

const child1 = new TreeNode("Child1");
const child2 = new TreeNode("Child2");
const parent1 = new TreeNode("Parent1", child1, child2);
const parent2 = new TreeNode("Parent2");
const grandparent = new TreeNode("Grandparent", parent1, parent2);

const result: string[] = [];

function postorderTraversal(node: TreeNode | null): void {
    if (node !== null) {
        postorderTraversal(node.left);
        postorderTraversal(node.right);
        result.push(node.name);
    }
}

// Call postorderTraversal on grandparent and print result
// Your code here

Hint

Call postorderTraversal(grandparent) and then print result with console.log(result).