##### ###### ##### ### # # ### # # ###### ## ## ## ## ## ## ## # # # # # ## ##### #### ##### # # # # # # # #### ## # ## ## ## ## # # # # # ## ## # ###### ## ### # ### # ######
##### ###### ##### ### # # ### # # ###### ## ## ## ## ## ## ## # # # # # ## ##### #### ##### # # # # # # # #### ## # ## ## ## ## # # # # # ## ## # ###### ## ### # ### # ######
The core idea
Use a stack of numbers. Scan left to right.
If you see a number, push it onto the stack.
If you see an operator, pop two numbers, apply the operator, and push the result back.
The second-to-top number is the left operand, the top is the right operand. This matters for subtraction and division.
At the end, the stack contains exactly one number: the answer.
