What is the time complexity of it?
Its hard to tell what the "complexity" of a loop without knowing what each iteration performs, but what we can do is count how many times each loop iterates:


  for (int i = 1; i &lt; n; i++)


You correctly stated this loop iterates $n$ times.


  for (int i = 0; i &lt; n; i+= 2)


The above loop iterates until $i \geq n$, or in other words, $k$ times, when $i+2k = n \rightarrow 0+2k = n$ (since you initiate $i=0$ and increment it by $2$). Solve the trivial equation, and you get $k=\frac{n}{2}$, which is $O(n)$ iterations.


  for (int j = 1; j &lt; n; j*= 2)


Same as the second time, we must solve $2^k = n$, since you multiply $j=1$ by $2$ each iteration. Solve and get $k=\log(n)$