题解 | #四则运算#

import java.util.*;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String str = sc.nextLine();
        System.out.println(calculate(str));
    }

    public static int calculate(String str) {

        // 处理特殊括号,注意替换后的值，不要写错了。且replace后要重新赋值给str
        str = str.replaceAll("\\[", "(").replaceAll("\\{", "(").replaceAll("]",
                ")").replaceAll("}", ")").replaceAll(" ", "");

        //初始化两个栈，分别存放运算术和运算符
        Stack<Integer> nums = new Stack<>();
        Stack<Character> opts = new Stack<>();
        char[] cs = str.toCharArray();
        int n = cs.length;
        boolean flagOpts = false;
        boolean isNegativeNumber = false;
        for (int i = 0; i < n; i++) {
            char c = cs[i];
            //判断负数、左括号、右括号、普通运算数、+-*/
            if (Character.isDigit(c)) {
                int  v = c - '0';
                int j = i + 1;
                while (j < n && Character.isDigit(cs[j])) {
                    v = v * 10 + cs[j++] - '0';
                }
                i = j - 1;
                if (isNegativeNumber) {
                    v = -v;
                    isNegativeNumber = false;
                }
                nums.add(v);
                flagOpts = false ;
            } else {
                if (flagOpts && c == '-') {
                    isNegativeNumber = true;
                    continue;
                }
                if (c == '(') {
                    opts.add(c);
                } else if (c == ')') {
                    while (opts.peek() != '(') {
                        cal(nums, opts);
                    }
                    opts.pop();
                    continue;
                } else {
                    while (!opts.isEmpty() && getPrecedence(c) <= getPrecedence(opts.peek())) {
                        cal(nums, opts);
                    }
                    opts.add(c);
                }
                flagOpts = true ;
            }
        }
        while (!opts.isEmpty()) {
            cal(nums, opts);
        }
        return nums.peek();
    }
    // 获取运算符的优先级
    public static int getPrecedence(char operator) {
        switch (operator) {
            case '+':
            case '-':
                return 1;
            case '*':
            case '/':
                return 2;
        }
        return 0;
    }

    public static void cal(Stack<Integer> nums, Stack<Character> opts) {
        if (nums.isEmpty() || nums.size() < 2) {
            return;
        }
        if (opts.isEmpty()) {
            return;
        }
        int b = nums.pop(), a = nums.pop();
        char op = opts.pop();
        int ans = 0;
        switch (op) {
            case '+' :
                ans = a + b;
                break;
            case '-':
                ans = a - b;
                break;
            case '*':
                ans = a * b;
                break;
            case '/':
                ans = a / b;
                break;
            default:
                break;
        }
        nums.add(ans);
    }

}