Custom Tree Sum

The phrase “custom tree sum” does not define which nodes or operation count, so ask for the exact rule, an example, tree shape, and whether values may be negative. Once confirmed, use a postorder DFS invariant: helper(node) returns the aggregate required for node's subtree. The base case for null is the identity element, usually zero for addition. Compute child aggregates first, apply the stated custom rule at the node, and return the result upward; a global accumulator is appropriate only when the desired value is not the same as the returned subtree aggregate. State whether a wide numeric type is needed. Do not assume a binary tree or ordinary sum from the title. A full traversal is O(n) time and O(height) recursion space.