150. Evaluate Reverse Polish Notation

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are `+`, `-`, `*`, `/`. Each operand may be an integer or another expression.

Note:

• Division between two integers should truncate toward zero.
• The given RPN expression is always valid. That means the expression would always evaluate to a result and there won't be any divide by zero operation.

Example 1:

```Input: ["2", "1", "+", "3", "*"]
Output: 9
Explanation: ((2 + 1) * 3) = 9
```

Example 2:

```Input: ["4", "13", "5", "/", "+"]
Output: 6
Explanation: (4 + (13 / 5)) = 6
```

Example 3:

```Input: ["10", "6", "9", "3", "+", "-11", "*", "/", "*", "17", "+", "5", "+"]
Output: 22
Explanation:
((10 * (6 / ((9 + 3) * -11))) + 17) + 5
= ((10 * (6 / (12 * -11))) + 17) + 5
= ((10 * (6 / -132)) + 17) + 5
= ((10 * 0) + 17) + 5
= (0 + 17) + 5
= 17 + 5
= 22
```

150. Evaluate Reverse Polish Notation
``````struct Solution;

impl Solution {
fn eval_rpn(tokens: Vec<String>) -> i32 {
let mut stack = vec![];
for tok in tokens {
match tok.as_ref() {
"+" => {
let right = stack.pop().unwrap();
let left = stack.pop().unwrap();
stack.push(left + right);
}
"-" => {
let right = stack.pop().unwrap();
let left = stack.pop().unwrap();
stack.push(left - right);
}
"*" => {
let right = stack.pop().unwrap();
let left = stack.pop().unwrap();
stack.push(left * right);
}
"/" => {
let right = stack.pop().unwrap();
let left = stack.pop().unwrap();
stack.push(left / right);
}
_ => {
stack.push(tok.parse::<i32>().unwrap());
}
}
}
stack[0]
}
}

#[test]
fn test() {
let tokens = vec_string!["2", "1", "+", "3", "*"];
let res = 9;
assert_eq!(Solution::eval_rpn(tokens), res);
let tokens = vec_string!["4", "13", "5", "/", "+"];
let res = 6;
assert_eq!(Solution::eval_rpn(tokens), res);
let tokens = vec_string!["10", "6", "9", "3", "+", "-11", "*", "/", "*", "17", "+", "5", "+"];
let res = 22;
assert_eq!(Solution::eval_rpn(tokens), res);
}
``````