  Up to now, relatively few exponential quantum speed-ups have been achieved.
Out of them, the welded tree problem (Childs, Cleve, Deotto, Farhi, Gutmann,
and Spielman'2003) is one of the unusual examples, as the exponential speed-up
is attained by a quantum walk. In this paper, we give a very short proof of the
optimal linear hitting time for this problem by a discrete-time quantum walk,
which is based on a simple modification of the electric quantum walk framework.
The same technique can be applied to other 1-dimensional hierarchical graphs,
yielding results similar to (Balasubramanian, Li, and Harrow'2023).
