Skip to main content

12. Symmetric Tree

easyAsked at CircleCI

Determine whether a binary tree is a mirror of itself.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

Given the root of a binary tree, return true if it is a mirror of itself (symmetric around its center). Solve it both recursively and iteratively.

Constraints

  • 1 <= number of nodes <= 1000
  • -100 <= Node.val <= 100

Examples

Example 1

Input
root = [1,2,2,3,4,4,3]
Output
true

Example 2

Input
root = [1,2,2,null,3,null,3]
Output
false

Approaches

1. BFS level compare

Collect each level and check it is a palindrome.

Time
O(n)
Space
O(n)
// Walk level by level, push vals including nulls, check level === level.reverse()

Tradeoff:

2. Mirror DFS

Recurse on (left.left, right.right) and (left.right, right.left).

Time
O(n)
Space
O(h)
function isSymmetric(root) {
  function mirror(a, b) {
    if (!a && !b) return true;
    if (!a || !b) return false;
    return a.val === b.val && mirror(a.left, b.right) && mirror(a.right, b.left);
  }
  return !root || mirror(root.left, root.right);
}

Tradeoff:

CircleCI-specific tips

CircleCI graders care about clean recursion bases — sloppy null handling here kills your symmetry signal.

Solve it now

Free. No sign-up. Python and JavaScript run instantly in your browser.

Output

Press Run or Cmd+Enter to execute

Practice these live with InterviewChamp.AI

Drill Symmetric Tree and other CircleCI interview questions under real-loop conditions with instant feedback on your reasoning, complexity claims, and code.

Practice these live with InterviewChamp.AI →