Solution2.py

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right


class Solution:
    def getDirections(
        self, root: Optional[TreeNode], startValue: int, destValue: int
    ) -> str:
        def dfs(node: Optional[TreeNode], x: int, path: List[str]):
            if node is None:
                return False
            if node.val == x:
                return True
            path.append("L")
            if dfs(node.left, x, path):
                return True
            path[-1] = "R"
            if dfs(node.right, x, path):
                return True
            path.pop()
            return False

        path_to_start = []
        path_to_dest = []

        dfs(root, startValue, path_to_start)
        dfs(root, destValue, path_to_dest)
        i = 0
        while (
            i < len(path_to_start)
            and i < len(path_to_dest)
            and path_to_start[i] == path_to_dest[i]
        ):
            i += 1
        return "U" * (len(path_to_start) - i) + "".join(path_to_dest[i:])