Symmetric Tree

Given the root of a binary tree, check whether it is a **mirror of itself** (i.e., symmetric around its center).

Input: root = [1,2,2,3,4,4,3]

Output: true

The tree is symmetric.

Input: root = [1,2,2,null,3,null,3]

Output: false

Not symmetric — asymmetric subtrees.

Time

O(n)

Space

O(h)

#1 Symmetric tree

Input: [1,2,2,3,4,4,3]

Expected: true

#2 Asymmetric

Input: [1,2,2,null,3,null,3]

Expected: false

#3 Single node

Input: [1]

Expected: true

#4 Another symmetric

Input: [1,2,2,null,3,3,null]

Expected: true