-- Code
function f(s,i)j=i or 0return#s>j and s..f(s:reverse(),j+1)or''end

-- Test cases
print(f("Hello!"))
print(f("a"))
print(f("abcd"))
print(f("OK!"))
print(f("4815162342"))
print(f("PPCG"))
print(f("42"))