题解 | 简单表达式计算

use std::io::{self, *};

fn calculate(n1: i32, n2: i32, op: &char) -> i32 {
    match *op {
        '+' => n1 + n2,
        '-' => n2 - n1,
        '*' => n1 * n2,
        '/' => n2 / n1,
        _ => -1
    } 
}

fn parse_expr(line: &mut String) -> i32 {
    let mut num_stack = Vec::new();
    let mut op_stack = Vec::new();
    let mut current_num: Option<i32> = None;

    let mut line = format!("{} ", line);
    for c in line.chars() {
        if let Some(digit) = c.to_digit(10) {
            let n = current_num.get_or_insert(0);
            *n = *n * 10 + digit as i32
        } else {
            if let Some(num) = current_num.take() {
                num_stack.push(num);
            }
            match c {
                '+' | '-' => {
                    while let Some(op_on_top) = op_stack.pop() {
                        let num1 = num_stack.pop().unwrap();
                        let num2 = num_stack.pop().unwrap();
                        let res = calculate(num1, num2, &op_on_top);
                        num_stack.push(res);
                    }
                    op_stack.push(c);
                },
                '*' | '/' => {
                    while let Some(&op_on_top) = op_stack.last() {
                        if op_on_top == '*' || op_on_top == '/' {
                            let op = op_stack.pop().unwrap();
                            let num1 = num_stack.pop().unwrap();
                            let num2 = num_stack.pop().unwrap();
                            num_stack.push(calculate(num1, num2, &op));
                        } else {
                            break;
                        }
                    }
                    op_stack.push(c);
                },
                _ => continue,
            }
        }
    }
    while let Some(op) = op_stack.pop() {
        let num1 = num_stack.pop().unwrap();
        let num2 = num_stack.pop().unwrap(); 
        let res = calculate(num1, num2, &op );
        num_stack.push(res);
    }
    
    num_stack.pop().unwrap()
}

fn main() {
    let mut stdin = io::stdin();
    for line in stdin.lock().lines() {
        let mut line = line.unwrap();
        if line == "END" {
            break;
        }
        let res = parse_expr(&mut line);
        println!("{}", res);
    }
}