题解 | 简单计算器

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        String str = in.nextLine();
        in.close();

        Stack<Character> opStack = new Stack<>();
        Stack<Double> numStack = new Stack<>();

        for (int i = 0; i < str.length(); i++) {
            char ch = str.charAt(i);
            if (ch == ' ') {
                continue;
            }
            if (Character.isDigit(ch)) {
                // 解析完整整数
                double num = 0;
                while (i < str.length() && Character.isDigit(str.charAt(i))) {
                    num = num * 10 + (str.charAt(i) - '0');
                    i++;
                }
                i--; // 循环结束后会自增，这里先减回去
                numStack.push(num);
            } else if (ch == '(') {
                opStack.push(ch);
            } else if (ch == ')') {
                // 计算直到遇到左括号
                while (opStack.peek() != '(') {
                    double b = numStack.pop();
                    double a = numStack.pop();
                    char op = opStack.pop();
                    numStack.push(calc(op, a, b));
                }
                opStack.pop(); // 弹出 '('
            } else if (isOperator(ch)) {
                // 当前运算符优先级不高于栈顶时，先计算栈顶
                while (!opStack.isEmpty() && opStack.peek() != '(' &&
                        getPriority(opStack.peek()) >= getPriority(ch)) {
                    double b = numStack.pop();
                    double a = numStack.pop();
                    char op = opStack.pop();
                    numStack.push(calc(op, a, b));
                }
                opStack.push(ch);
            }
        }

        // 处理剩余运算符
        while (!opStack.isEmpty()) {
            double b = numStack.pop();
            double a = numStack.pop();
            char op = opStack.pop();
            numStack.push(calc(op, a, b));
        }

        // 输出结果
        System.out.printf("%.2f",numStack.pop());
    }

    public static boolean isOperator(char c) {
        return c == '+' || c == '-' || c == '*' || c == '/';
    }

    public static int getPriority(char c) {
        if (c == '+' || c == '-') {
            return 1;
        } else if (c == '*' || c == '/') {
            return 2;
        }
        return 0; // 括号优先级最低
    }

    public static double calc(char op, double a, double b) {
        switch (op) {
            case '+':
                return a + b;
            case '-':
                return a - b;
            case '*':
                return a * b;
            case '/':
                return a / b;
            default:
                throw new IllegalArgumentException("非法运算符: " + op);
        }
    }
}