Wiring for Adding Switched Closet Light
You could do either solution as long as you know the proper way to wire up mains electrical circuits. 

The solution on the bottom right is probably the preferred solution because you just have to add wires. If you go with the left side diagram you cannot just split the existing wire and expect it to nicely be spliced into inside the added switch box. There just would not be enough wire length available. 

If you do go with the left side solution make sure to split off the cable well below the new switch box location so there is adequate length to pull into and out the face of the new box. Then you would need to run a new longer wire from the lower outlet box back up to the new switch box.

So as you can see the right side solution is easier as you are just adding additional wires instead of disturbing existing connections.