Nodes With Even-Valued Grandparent

Pass ancestry downward

DFS from the root with parameters (node, parentValue, grandparentValue) or node references. When visiting a node, add its value if a grandparent exists and that grandparent value is even. Recurse into each child, shifting the current node into the parent position and parent into grandparent position. This avoids searching upward from every node.

The invariant is that the two carried values are exactly the ancestors one and two edges above the current node. Each tree node is visited once, so time is O(n). Recursion uses O(h) auxiliary space for height h; an explicit stack has the same asymptotic bound. Test a tree with fewer than three levels, a root with even value but no grandchildren, negative even values, skewed trees, and multiple branches. If the API uses nullable nodes, stop immediately at null. Do not accidentally test the parent instead of grandparent, and do not add an ancestor's value when the task asks for the descendant's value.