Symmetric example
Input
root = TreeNode(1, TreeNode(2, TreeNode(3), TreeNode(4)), TreeNode(2, TreeNode(4), TreeNode(3)))
Output
True
Left and right subtrees are mirrors of each other.
Full lesson preview
Check if a tree is symmetric
Determine whether a binary tree is a mirror of itself (symmetric around its center).
Python practice15 minTrees & Binary TreesIntermediateLast updated March 29, 2026
Problem statement
Given the root of a binary tree, determine if it is symmetric around its center. A binary tree is symmetric if the left subtree is a mirror reflection of the right subtree.
Return True if the tree is symmetric, otherwise return False.
You may use a recursive approach (comparing pairs of nodes) or an iterative approach with a queue/stack.
Task
Implement a function that checks whether a binary tree is symmetric (mirror) using recursion or iteration.
Examples
Input format
A binary tree constructed using TreeNode(val, left, right). The root may be None.
Output format
Return True if the tree is symmetric, otherwise False.
Constraints
- Number of nodes can be zero (empty tree) or more.
- Aim for O(n) time and O(h) extra space (recursion stack) or O(n) with iterative approach.
Samples
Sample 1
Input
TreeNode(1, TreeNode(2, None, TreeNode(3)), TreeNode(2, None, TreeNode(3)))
Output
False
Structure not mirrored even though some values match.