Shade a loop that goes through every number. The numbers cut the loop into paths, one between each two numbers next to each other on the loop.
Each path turns at most once (turning on a number is free) and holds at least one cell, so no number is 0.
Each number counts the cells between it and the next number along the loop, every number counting the same way round. A "?" is a number whose count isn't told.
Two paths can't touch orthogonally: shaded cells are only side by side where the loop runs from one to the other.