‹ DS&A interview · Socratic
DSA · Trees · #176

Symmetric Tree

Module 44 · difficulty 2/5·⏱ 30:00starts on first keystroke

Given the `root` of a binary tree, return `true` if the tree is a mirror of itself (symmetric around its center), otherwise return `false`. A tree is symmetric when the left subtree is a mirror reflection of the right subtree. Implement the function `isSymmetric(root)`. Each node is `{ val, left, right }`; an empty tree is represented by `null`.

Examples
  • root = [1,2,2,3,4,4,3] → true — The two subtrees of the root mirror each other exactly.
  • root = [1,2,2,null,3,null,3] → false — The 3s are positioned on the same side, breaking mirror symmetry.
  • root = [] → true — An empty tree is trivially symmetric.
Constraints
  • · The number of nodes is in the range [0, 1000].
  • · -100 <= Node.val <= 100
Session phases
A · Clarify
B · Approach
C · Complexity
D · Edges
E · Code
F · Tradeoff
G · Score
Phase A — Clarify
Ask questions about input bounds, types, and edge constraints.
Ask the coach clarifying questions about the problem.
When you've covered this phase, advance to the next.