You are given the root of a binary tree whose node values are unique, together with the values of a starting node and a destination node. Return the directions that move from the starting node to the destination.

Use L to move from a node to its left child, R to move to its right child, and U to move to its parent. The returned string must describe one valid route, with no extra steps.

The tree is supplied in level-order form. A null entry means that the corresponding child is absent. Both requested values occur in the tree.