Evaluate Reverse Polish Notation (leetcode #150)

lib.rs

pub fn eval_rpn(tokens: &[&str]) -> i32 {
    tokens
        .iter()
        .fold(vec![], |mut stack, token| match *token {
            op @ ("+" | "-" | "*" | "/") => {
                let rhs = stack.pop().unwrap();
                let lhs = stack.pop().unwrap();

                stack.push(match op {
                    "+" => lhs + rhs,
                    "-" => lhs - rhs,
                    "*" => lhs * rhs,
                    "/" => lhs / rhs,
                    _ => unreachable!(),
                });

                stack
            }
            n => {
                stack.push(n.parse().unwrap());
                stack
            }
        })
        .pop()
        .unwrap()
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn example_1() {
        let result = eval_rpn(&["2", "1", "+", "3", "*"]);
        assert_eq!(result, 9);
    }

    #[test]
    fn example_2() {
        let result = eval_rpn(&["4", "13", "5", "/", "+"]);
        assert_eq!(result, 6);
    }

    #[test]
    fn example_3() {
        let result = eval_rpn(&[
            "10", "6", "9", "3", "+", "-11", "*", "/", "*", "17", "+", "5", "+",
        ]);
        assert_eq!(result, 22);
    }
}