##### ###### ##### ### # # ### # # ###### ## ## ## ## ## ## ## # # # # # ## ##### #### ##### # # # # # # # #### ## # ## ## ## ## # # # # # ## ## # ###### ## ### # ### # ######
##### ###### ##### ### # # ### # # ###### ## ## ## ## ## ## ## # # # # # ## ##### #### ##### # # # # # # # #### ## # ## ## ## ## # # # # # ## ## # ###### ## ### # ### # ######
Walkthrough and analysis
Trace ["4", "13", "5", "/", "+"].
Read "4": push 4. Stack: [4].
Read "13": push 13. Stack: [4, 13].
Read "5": push 5. Stack: [4, 13, 5].
Read "/": pop 5 and 13. Compute 13 / 5 = 2. Push 2. Stack: [4, 2].
Read "+": pop 2 and 4. Compute 4 + 2 = 6. Push 6. Stack: [6].
Return 6.
Single pass through tokens. time. Stack holds at most numbers. space.
