题解 | #四则运算#

import java.util.*;

// 类似的题目：https://www.nowcoder.com/discuss/454321545557635072?sourceSSR=users
// mark:
// 1. 将[ { 都替换为 )，使用replace函数 replaceAll是正则替换 同时需要重新赋值回去 否则值不变
// 2. 循环从[0, length]，字符串末尾添加一个标识符# 防止最后一位是数字的情况 保证最后一个数字能够入栈
// 3. 由于2步骤，所以在判断输入的字符类型时，最后不能是else，需要是else-if (c == ')')
// 4. 注意判断是否是负数
// 5. 各个符号的优先级

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        while (in.hasNextLine()) {
            String str = in.nextLine();
            int length = str.length();
            str = str.replace("[", "(");
            str = str.replace("]", ")");
            str = str.replace("{", "(");
            str = str.replace("}", ")");
            Stack<Integer> numStack = new Stack<>();
            Stack<Character> opStack = new Stack<>();
            String numStr = "";
            // 从 0 到 length
            for (int i = 0; i <= length; i++) {
                // 字符串末尾添加标识符 否则 如果最后一个字符是数字 那么它将无法入栈
                char c = (i == length) ? '#' : str.charAt(i);
                if (Character.isDigit(c)) {
                    numStr += c;
                } else {
                    if (numStr.length() > 0) {
                        numStack.push(Integer.valueOf(numStr));
                        numStr = "";
                    }

                    if (c == '+' || c == '-') {
                        // 注意判断是否是 负数
                        if (c == '-' && (i == 0
                                || str.charAt(i - 1) == '('
                        )) {
                            numStr = "-";
                            continue;
                        }

                        while (!opStack.isEmpty() &&
                                ((opStack.peek() == '+') || (opStack.peek() == '-') ||
                                        (opStack.peek() == '*') || (opStack.peek() == '/'))) {
                            char c1 = opStack.pop();
                            int num2 = numStack.pop();
                            int num1 = numStack.pop();
                            numStack.push(calc(c1, num1, num2));
                        }
                        // 压入 + 或 -
                        opStack.push(c);
                    } else if (c == '*' || c == '/') {
                        opStack.push(c);
                    } else if (c == '(') {
                        opStack.push(c);
                    } else if (c == ')'){
                        while (!opStack.isEmpty() &&
                                (opStack.peek() != '(')) {
                            char c1 = opStack.pop();
                            int num2 = numStack.pop();
                            int num1 = numStack.pop();
                            numStack.push(calc(c1, num1, num2));
                        }
                        // 弹出(
                        opStack.pop();
                    }
                }
            }

            while (!opStack.isEmpty()) {
                char c = opStack.pop();
                int num2 = numStack.pop();
                int num1 = numStack.pop();
                numStack.push(calc(c, num1, num2));
            }

            System.out.println(numStack.pop());
        }
    }

    public static int calc(char c, int num1, int num2) {
        switch (c) {
            case '+':
                return num1 + num2;
            case '-':
                return num1 - num2;
            case '*':
                return num1 * num2;
            case '/':
                return num1 / num2;
            default:
                return 0;
        }
    }
}