Check if Two Trees are Symmetric in DSA Typescript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of symmetric check on two trees

What is the output of the following TypeScript code that checks if two binary trees are symmetric?

DSA Typescript

class TreeNode {
  val: number;
  left: TreeNode | null;
  right: TreeNode | null;
  constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
    this.val = val === undefined ? 0 : val;
    this.left = left === undefined ? null : left;
    this.right = right === undefined ? null : right;
  }
}

function isSymmetric(t1: TreeNode | null, t2: TreeNode | null): boolean {
  if (!t1 && !t2) return true;
  if (!t1 || !t2) return false;
  return t1.val === t2.val && isSymmetric(t1.left, t2.right) && isSymmetric(t1.right, t2.left);
}

const treeA = new TreeNode(1, new TreeNode(2, null, new TreeNode(3)), new TreeNode(2, null, new TreeNode(3)));
const treeB = new TreeNode(1, new TreeNode(2, null, new TreeNode(3)), new TreeNode(2, new TreeNode(3), null));

console.log(isSymmetric(treeA, treeB));

A TypeError

B false

C SyntaxError

D true

💡 Hint

Check if the left subtree of one tree matches the right subtree of the other tree recursively.

The function checks if two trees are mirror images. treeA and treeB differ in structure on the right subtree of the root's children, so they are not symmetric.

🧠 Conceptual

intermediate

Understanding symmetry in binary trees

Which of the following best describes when two binary trees are symmetric?

A They have the same number of nodes regardless of structure.

B They have the same values in the same positions.

C One tree is the mirror image of the other, with matching node values.

D They have identical left subtrees but different right subtrees.

💡 Hint

Symmetry means one tree looks like the mirror reflection of the other.

Two trees are symmetric if one is the mirror image of the other, meaning the left subtree of one matches the right subtree of the other and vice versa, with equal node values.

🔧 Debug

advanced

Identify the error in symmetric tree check

What error will this TypeScript code produce when checking if two trees are symmetric?

DSA Typescript

function isSymmetric(t1: TreeNode | null, t2: TreeNode | null): boolean {
  if (!t1 && !t2) return true;
  if (!t1 || !t2) return false;
  return t1.val === t2.val && isSymmetric(t1.left, t2.left) && isSymmetric(t1.right, t2.right);
}

A Returns false for symmetric trees due to wrong recursive calls

B SyntaxError due to missing return statement

C No error, returns correct result

D TypeError because of null property access

💡 Hint

Check if the recursive calls compare the correct subtrees for symmetry.

The code compares left subtree with left subtree and right with right, which checks equality but not symmetry (mirror). It will return false for symmetric trees.

❓ Predict Output

advanced

Output of symmetric check with null trees

What is the output of this TypeScript code when both trees are null?

DSA Typescript

function isSymmetric(t1: TreeNode | null, t2: TreeNode | null): boolean {
  if (!t1 && !t2) return true;
  if (!t1 || !t2) return false;
  return t1.val === t2.val && isSymmetric(t1.left, t2.right) && isSymmetric(t1.right, t2.left);
}

console.log(isSymmetric(null, null));

A false

B TypeError

C SyntaxError

D true

💡 Hint

Two empty trees are symmetric by definition.

If both trees are null, the function returns true immediately as they are symmetric (empty trees).

🚀 Application

expert

Number of symmetric subtree pairs in two trees

Given two binary trees, how many pairs of subtrees (one from each tree) are symmetric according to the isSymmetric function below? Consider these trees: Tree1: 1 / \ 2 3 / \ 4 5 Tree2: 1 / \ 3 2 / \ 5 4 Use the isSymmetric function: function isSymmetric(t1: TreeNode | null, t2: TreeNode | null): boolean { if (!t1 && !t2) return true; if (!t1 || !t2) return false; return t1.val === t2.val && isSymmetric(t1.left, t2.right) && isSymmetric(t1.right, t2.left); }

A 5

B 4

C 3

D 6

💡 Hint

Count all subtree pairs including the root and leaf nodes that are mirror images.

The symmetric subtree pairs are: - Leaf nodes 4 and 4 - Leaf nodes 5 and 5 - Subtrees rooted at 2 and 2 - Subtrees rooted at 3 and 3 - The whole trees rooted at 1 and 1 Total 5 unique pairs.