Check if Two Trees are Symmetric in DSA Typescript - Build from Scratch

Check if Two Trees are Symmetric

📖 Scenario: Imagine you have two family trees represented as binary trees. You want to check if these two trees are mirror images of each other, meaning they are symmetric around their center.

🎯 Goal: Build a TypeScript program that creates two binary trees, sets a helper variable, writes a function to check if the two trees are symmetric, and finally prints the result.

📋 What You'll Learn

Create two binary trees using the TreeNode class with exact values

Create a helper variable called result initialized to true

Write a function called isSymmetric that takes two TreeNode parameters and returns a boolean

Use recursion inside isSymmetric to check if the trees are symmetric

Print the final result which is true if symmetric, otherwise false

💡 Why This Matters

🌍 Real World

Checking if two trees are symmetric is useful in graphics, data organization, and comparing hierarchical data structures.

💼 Career

Understanding tree symmetry helps in coding interviews and software roles involving data structures and algorithms.

1

Create two binary trees

Create a class called TreeNode with val, left, and right properties. Then create two trees called tree1 and tree2 with these exact nodes:

tree1: root value 1, left child 2, right child 3
tree2: root value 1, left child 3, right child 2

DSA Typescript

class TreeNode {
  val: number;
  left: TreeNode | null;
  right: TreeNode | null;
  constructor(val: number, left: TreeNode | null = null, right: TreeNode | null = null) {
    this.val = val;
    this.left = left;
    this.right = right;
  }
}

// Create tree1 and tree2 below
// Your code here

Hint

Define the TreeNode class first. Then create tree1 with root 1, left 2, right 3. Create tree2 with root 1, left 3, right 2.

2

Add a helper variable

Create a boolean variable called result and set it to true. This will store if the trees are symmetric or not.

DSA Typescript

class TreeNode {
  val: number;
  left: TreeNode | null;
  right: TreeNode | null;
  constructor(val: number, left: TreeNode | null = null, right: TreeNode | null = null) {
    this.val = val;
    this.left = left;
    this.right = right;
  }
}

const tree1 = new TreeNode(1, new TreeNode(2), new TreeNode(3));
const tree2 = new TreeNode(1, new TreeNode(3), new TreeNode(2));

// Create a boolean variable called result and set it to true
// Your code here

Hint

Just write let result = true; below the tree creation.

3

Write the symmetric check function

Write a function called isSymmetric that takes two parameters node1 and node2 of type TreeNode | null. Use recursion to check if the two trees are mirror images. Return true if symmetric, otherwise false. Use these rules:
- If both nodes are null, return true
- If one is null and the other is not, return false
- If values differ, return false
- Recursively check node1.left with node2.right and node1.right with node2.left

DSA Typescript

class TreeNode {
  val: number;
  left: TreeNode | null;
  right: TreeNode | null;
  constructor(val: number, left: TreeNode | null = null, right: TreeNode | null = null) {
    this.val = val;
    this.left = left;
    this.right = right;
  }
}

const tree1 = new TreeNode(1, new TreeNode(2), new TreeNode(3));
const tree2 = new TreeNode(1, new TreeNode(3), new TreeNode(2));

let result = true;

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

Hint

Use the rules given to write the recursive function. Then assign result by calling isSymmetric(tree1, tree2).

4

Print the result

Print the value of the variable result to show if the two trees are symmetric or not.

DSA Typescript

class TreeNode {
  val: number;
  left: TreeNode | null;
  right: TreeNode | null;
  constructor(val: number, left: TreeNode | null = null, right: TreeNode | null = null) {
    this.val = val;
    this.left = left;
    this.right = right;
  }
}

const tree1 = new TreeNode(1, new TreeNode(2), new TreeNode(3));
const tree2 = new TreeNode(1, new TreeNode(3), new TreeNode(2));

let result = true;

function isSymmetric(node1: TreeNode | null, node2: TreeNode | null): boolean {
  if (node1 === null && node2 === null) return true;
  if (node1 === null || node2 === null) return false;
  if (node1.val !== node2.val) return false;
  return isSymmetric(node1.left, node2.right) && isSymmetric(node1.right, node2.left);
}

result = isSymmetric(tree1, tree2);

// Print the result below
// Your code here

Hint

Use console.log(result); to print the final answer.