https://en.wikipedia.org/wiki/Reverse_Polish_notation
Reverse Polish Notation(RPN) is a stack based mathimatical notation which the operators follow their operands.
For example the expression 25 17 + => 42 will add 25 and 17, with is equivilant to 25 + 17 in tradisional notation.
When performing the expression, a stack is used to store the current values
When adding two numbers,
( 25 17 + )
25 is pushed on the stack
stack: 25
17 is pushed on the stack
stack: 25 17
perform + operation
17 is popped from the stack
25 is popped from the stack
result 42 is pushed on the stack
end of expression
remaining stack prined 42
If there is more than one value left on the stack, the stack entire stack will be printed
( 4 14 6 + )
results in
4 20