USA: +1 (725) 712-1730 
Sign in
Write a program using the language Rust.
## Description
Write a [postfix](https://en.wikipedia.org/wiki/Reverse_Polish_notation) expression evaluator. An expression consists of operands and operators. An operand is a signed integer (`isize`). An operator is `+`, `-`, or `*` with their common semantics. An expression is valid if it can be evaluated to a signed integer.
For example, the following are valid expressions:
```
-100
1 2 +
1 2 3 + *
```
The following expressions are invalid:
```
// empty expression
-1 -2
1 2 + *
```
## Public API
Your program must provide the following public API.
```
pub enum Operator {
// `+`
Add,
// `-`
Sub,
// `*`
Mul,
}
pub enum Token {
Operator(Operator),
Operand(isize),
}
/// Evaluates the postix expression.
///
/// Input: a postfix expression, where each element contains an operator or operand.
/// Returns: if the postfix expression is valid, returns `Some(value)`;
/// otherwise, returns `None`.
pub fn eval(tokens: &[Token]) -> Option<isize> {
// TODO
unimplemented!();
}
Expert Answer
Editable code:
fn main() {
let data1 = Token::Operand(3);
let data2 = Token::Operand(2);
let sign1 = Operator::Add;
let data3 = Token::Operator(sign1);
eval(&[data1, data2, data3]);
}
pub enum Operator {
Add,
Sub,
Mul,
}
pub enum Token {
Operator(Operator),
Operand(isize),
}
pub fn eval(tokens11: &[Token]) -> Option<isize> {
let mut v1: Vec<isize> = Vec::new();
for i1 in tokens11 {
match i1 {
&Token::Operator(ref op1) => {
if v1.len()>1 {
let x1 = v1.pop().unwrap();
let y1 = v1.pop().unwrap();
match op1 {
&Operator::Add => v1.push(x1+y1),
&Operator::Sub => v1.push(y1-x1),
&Operator::Mul => v1.push(x1*y1),
};
} else {
return None;
}
},
&Token::Operand(num) => v1.push(num),
};
}
if v1.len()==1 {
return v1.pop()
} else {
return None
}
}
