nice code but not really correct. This gives an output of [H, o, w, , c, a, n, , m, i, r, r, o, r, s, , b, e, , r, e, a, l, , i, f, , o, u, r, , e, y, e, s, , a, r, e, n, ', t, , r, e, a, l]

is O(n^2) here ?

Sorry, it really does work :(

Could you explain this? thanks!

Is it possible to test the code on Eclipse?, I'm new to programming, and I can't run the code.

Cool solution, thank you!

can u explain the return statement

At the return statment we could also do:
return builder.substring(0, builder.length() - 1);

second loop ch should be converted to lowe case.