Given the roots of two complete binary trees with identical structure, modify the node values of the second tree so that each node equals the sum of all node values in the subtree rooted at the same position in the first tree (including that corresponding root). Provide an algorithm that runs in linear time relative to the number of nodes and uses at most O(h) extra space, where h is tree height. Describe recursion vs. iteration choices, how you verify shape identity, and how you would test edge cases (single node, skewed last level, large values).