101. Symmetric Tree

https://leetcode.com/problems/symmetric-tree/

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

    1
   / \
  2   2
 / \ / \
3  4 4  3

But the following [1,2,2,null,3,null,3] is not:

    1
   / \
  2   2
   \   \
   3    3

Follow up: Solve it both recursively and iteratively

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Iterative - level order - queue based
Offer 2 nodes, and remove 2 nodes at a time

Offer nulls as well, don't filter them out

Offer mirror nodes together
n1.left, n2.right
n1.right, n2.left
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Time - O(n)
Space - O(n)
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