it looks same with my own XD

Grab pen and paper, and draw the path:

n: up
s: down
e: right
w: left

return true if the walk the app gives you will take you exactly ten minutes (you don't want to be early or late!) and will, of course, return you to your starting point.

why it doesn't?
what the mean of starting point

if('n') is always true as well as other expressions of a kind

The first one comes back to the starting point and the other doesn't.

what is different between this?

Assert::That(isValidWalk(V{'s','e','w','n','n','s','e','w','n','s'}), Equals(true));
Assert::That(isValidWalk(V{'n','s','n','s','n','s','n','s','n','n'}), Equals(false));