3.1

1. Change `i--` to `n--`
2. `i<n` to `-i<n`
3. (Unfortunately invalid answer because I wasn't checking with compiler before seeing other answers)

3.2.1

    int n = 20 
to 

    uint n = 20
(Unfortunately not standard...)

3.2.2

    int n = -20L;

    for(i = -1; i%n; i--)

prints 19 + signs, but can't take full credit for it because it wouldn't have worked with uint.