I have a some elements in a queue (for example
/ - 4 2 + 4 5
You try to understand a polish notation that is a form of notation for logic, arithmetic, and algebra.
The expression for adding the numbers 4 and 5 is, in prefix/polish notation, written
+ 4 5 rather than
4 + 5.
The expression for subtracting the numbers 4 and 2 is, in prefix/polish notation, written
- 4 2 rather than
4 - 2.
Then, operations can be composed.
op3 (op1 m1 n1) (op2 m2 n2), which can be interpreted as
(m1 op1 n1) op3 (m2 op2 n2), where
op3 can be
Parentheses are optional and the previous polish notation can be written
op3 op1 m1 n1 op2 m2 n2
The easiest way to understand it, is to use a tree. A tree will also help you to better understand the recursive algorithm that are used to evaluate such notations. On such a tree, any operation will be considered as a node, and numbers are leaves.
To evaluate an expression, one needs: