Symmetric Tree

Return True if the tree is symmetric: its left subtree is the mirror image of its right subtree.

The tests write a tree as a level-order list (row by row, left to right) with None for a missing child: [3, 9, 20, None, None, 15, 7] is a root 3 with children 9 and 20, where 20 has children 15 and 7.

For [1, 2, 2, 3, 4, 4, 3] the answer is True; for [1, 2, 2, None, 3, None, 3] it is False.

Examples

Example 1

Input:

is_symmetric(root)  # root = [1, 2, 2, 3, 4, 4, 3]

Output:

True

Example 2

Input:

is_symmetric(root)  # root = [1, 2, 2, None, 3, None, 3]

Output:

False

Constraints

• 0 <= number of nodes <= 100000
• The tree is not deeper than 800 levels.