题解 | #表达式求值#

import java.util.*;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    /**
     * 中缀表达式转后缀表达式
     */
    public static String toSuffix(String expression) {
        // 操作符栈
        Deque<Character> stack = new ArrayDeque<>();
        StringBuilder suffix = new StringBuilder();  // 后缀表达式字符串
        int i = 0;
        while (i < expression.length()) {
            char ch = expression.charAt(i);
            switch (ch) {
                case '+':
                case '-':
                    if (ch == '-') {
                        if (i == 0) {
                            suffix.append(0).append(" ");
                            stack.push('-');
                            i++;
                            break;
                        } else {
                            char c = expression.charAt(i - 1);
                            if (c == '(') {
                                suffix.append(0).append(" ");
                                stack.push('-');
                                i++;
                                break;
                            }
                        }
                    }
                    while (!stack.isEmpty() && stack.peek() != '(') {
                        suffix.append(stack.pop()).append(" ");
                    }
                    stack.push(ch);
                    i++;
                    break;
                case '*':
                case '/':
                    while (!stack.isEmpty() && (stack.peek() == '*' || stack.peek() == '/')) {
                        suffix.append(stack.pop()).append(" ");
                    }
                    stack.push(ch);
                    i++;
                    break;
                case '(':
                    stack.push(ch);
                    i++;
                    break;
                case ')':
                    Character symbol = stack.pop();
                    while (symbol != null && symbol != '(') {
                        suffix.append(symbol).append(" ");
                        symbol = stack.pop();
                    }
                    i++;
                    break;
                default:  // 遇到数字
                    while (i < expression.length() && Character.isDigit(ch)) {
                        suffix.append(ch);
                        i++;
                        if (i < expression.length()) {
                            ch = expression.charAt(i);
                        }
                    }
                    suffix.append(" ");  // 添加空格作为数值之间的分隔符
            }
        }
        while (!stack.isEmpty()) {
            suffix.append(stack.pop()).append(" ");
        }
        return suffix.toString();
    }

    /**
     * 计算后缀表达式的值
     */
    public static int calculate(String suffixExpression) {
        // 操作数栈
        Deque<Integer> stack = new ArrayDeque<>();
        int ans = 0;
        for (int i = 0; i < suffixExpression.length(); i++) {
            char ch = suffixExpression.charAt(i);
            if (Character.isDigit(ch)) {
                ans = 0;
                while (ch != ' ') {
                    // 数字字符转为整数值
                    ans = ans * 10 + ch - '0';
                    ch = suffixExpression.charAt(++i);
                }
                stack.push(ans);  // 整数对象入栈
            } else {
                if (ch != ' ') {
                    // 出栈两个操作数，注意出栈次序
                    int y = stack.pop();
                    int x = stack.pop();
                    switch (ch) {
                        case '+':
                            ans = x + y;
                            break;
                        case '-':
                            ans = x - y;
                            break;
                        case '*':
                            ans = x * y;
                            break;
                        case '/':
                            ans = x / y;
                            break;
                    }
                    stack.push(ans);  // 运算结果入栈
                }
            }
        }
        return stack.pop();
    }

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String expression = sc.next();
        String suffixExpression = toSuffix(expression);
        int ans = calculate(suffixExpression);
        System.out.println(ans);
    }
}