In-place merge two height-balanced BSTs

Given two height-balanced binary search trees, in-place merge them into a single balanced binary search tree. For each node of a height-balanced tree, the difference between its left and right subtree height is at most 1.

Netherlands 1959 Europa. MH/MH set.

Given a binary tree and two tree pointers, x and y, write an efficient algorithm to check if they lie on the same root-to-leaf path in the binary tree. In other words, determine whether x is an ancestor of y, or x is a descendant of y.