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

Rust Solution

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);
}

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