Same Tree & Symmetric Tree

Two classic questions that turn out to be the same algorithm, so they share one lesson:

Example

Same Tree:      p = [1, 2, 3],  q = [1, 2, 3]   -> true
                p = [1, 2],     q = [1, null, 2] -> false   (same values, different shape)
Symmetric Tree: root = [1, 2, 2, 3, 4, 4, 3]     -> true
                root = [1, 2, 2, null, 3, null, 3] -> false

Here's the connection. To check two trees are the same, compare them position by position: the roots, then left with left and right with right. A tree is symmetric when its left subtree is the mirror of its right subtree — the same comparison, except the pairs are crossed: left with right, and right with left.