题解 | #四则运算#

class HelloWorld {
    public static void main(String[] args) {
       Scanner in = new Scanner(System.in);
        String s = in.nextLine();
      //预处理
        s = s.replaceAll("\\{", "(").replaceAll("\\}", ")")
                .replaceAll("\\[", "(").replaceAll("\\]", ")")
                .replaceAll("\\(\\+", "\\(0\\+").replaceAll("\\(\\-", "\\(0\\-");

        if (s.charAt(0) == '-') {
            s = '0' + s;
        }
//        优先级
        Map<Character, Integer> map = new HashMap<>();
        map.put('+', 1);
        map.put('-', 1);
        map.put('*', 2);
        map.put('/', 2);
      // 使用两个栈
        Stack<Character> op = new Stack<>();
        Stack<Integer> num = new Stack<>();

        for (int i = 0; i < s.length(); i++) {
            if (Character.isDigit(s.charAt(i))) {// 操作数
                int j = 0, k = i;
                while (k<s.length()&&Character.isDigit(s.charAt(k))) {
                    int g = s.charAt(k) - '0';
                    j = j * 10 + g;
                    k++;
                }
                i = k - 1;
                num.push(j);
            } else {//运算符
              // 括号单独处理
                if (s.charAt(i) == '(') {
                    op.push(s.charAt(i));
                } else if (s.charAt(i) == ')') {
                    while (op.peek()!='(') {
                        calc1(num,op);
                    }
                    op.pop();
                } else {//运算符
                    while (!op.isEmpty() && op.peek() != '(' && map.get(op.peek()) >= map.get(s.charAt(i))) {
                        calc1(num, op);
                    }
                    op.push(s.charAt(i));
                }
            }
        }

        while (!op.isEmpty()) {
            calc1(num,op);
        }
        System.out.println(num.peek());
    }
  
  static void calc1(Stack<Integer> num, Stack<Character> op) {
        Integer pop2 = num.pop();
        Integer pop1 = num.pop();
        Character p = op.pop();
        switch (p) {
            case '+':
                num.push(pop1 + pop2);
                break;
            case '-':
                num.push(pop1 - pop2);
                break;
            case '*':
                num.push(pop1 * pop2);
                break;
            case '/':
                num.push(pop1 / pop2);
                break;
        }
    }
}