Symmetric Tree

L7 Medium Trees

Concept

Mirror symmetry means comparing a "left side of the left" with a "right side of the right" — a doubled recursive comparison.

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

Examples

▸ root = [1, 2, 2, 3, 4, 4, 3]

→ true

▸ root = [1, 2, 2, nil, 3, nil, 3]

→ false

▸ root = []

→ true

Progressive Hints

Hint 1 · Nudge

Mirror symmetry means comparing mirror images of the two halves of the tree.

Hint 2 · Plan

Compare the left and right subtrees as mirrors: values equal, and mirror(left.left, right.right) together with mirror(left.right, right.left).

Hint 3 · Approach

isMirror(a, b): if both nil return true; if exactly one nil return false; if a.val != b.val return false; return isMirror(a.left, b.right) and isMirror(a.right, b.left). Return isMirror(root.left, root.right).

Output

// Run your code to see the output here.