Rules
Geradeweg rules: Draw a single, non-intersecting loops passing orthogonally by the center of some cells. The loop must pass by every numbered cell. The length of each segment passing through a numbered cell need to be the length of the clue. Hence, if the loop turns in the numbered clue, each branch needs to be the length of the clue, and if it goes straight the segment crossing the number is of the length of the clue.
A question mark can represent any number.
Example
Here is an example of a Geradeweg grid and its solution.
Solve the example online here.
History of Geradeweg
Geradeweg has been invented by Robert Vollmert (Germany). He posted some as soon as 2013 on the LMD portal.