Is the tree its own mirror?

Problem

A tree is symmetric if its left side is the mirror image of its right side, in shape and in values. Print YES or NO. An empty tree is symmetric.

Input: Line 1: the tree in level order, values separated by spaces, with null for a missing child.
Output: YES or NO.
Constraints: 0 ≤ number of nodes ≤ 3000
-1000 ≤ each value ≤ 1000

Example 1

Input

1 2 2 3 4 4 3

Output

YES

Symmetric.

Example 2

Input

1 2 2 null 3 null 3

Output

NO

The 3 values both sit on the right.

Try the problem first. Open one hint at a time.

Hint 1

Compare the left side of one half with the right side of the other.

Hint 2

Two missing nodes match. One missing node does not.

Write your Python code, run it on the examples and submit it to hidden tests.