题目

import java.util.Stack;

public class Code {
    public int calculate(String s) {
        Stack<Integer> data_stack = new Stack<>();
        Stack<Character> char_stack = new Stack<>();

        int length = s.length();
        int i = 0;
        while (i < length){
            char c = s.charAt(i);
            if (c == ' '){ //空格直接跳过
                i++;
            } else if (checkNumber(c)){ //如果是数字，处理如多个数字的情况 如333+1中的333
                int tmp = 0;
                while (i < length && checkNumber(s.charAt(i))){
                    tmp = tmp * 10 + (s.charAt(i) - '0');
                    i++;
                }
                data_stack.push(tmp);
            } else if (c == '('){ //左括号 直接入栈
                char_stack.push(c);
                i++;
            } else if (c == ')'){ //右括号 出栈计算 直到碰到'('为止
                while (!char_stack.isEmpty() && char_stack.peek() != '('){
                    fetchAndCol(data_stack,char_stack);
                }
                char_stack.pop(); //弹出'('
                i++;
            } else {
                //如果字符栈为空 或者 运算字符 优先级 大于 栈顶字符，则运算字符直接入栈
                if (char_stack.isEmpty() || checkChar(c,char_stack.peek())){
                    char_stack.push(c);
                } else {
                    //优先级: 运算字符 <= 栈顶字符 ,则拿数字栈的前两个数字和栈顶字符栈计算，并把结果压入数字栈
                    while (!char_stack.isEmpty() && !checkChar(c,char_stack.peek())){
                        fetchAndCol(data_stack,char_stack);
                    }
                    char_stack.push(c); //最后再把字符入字符栈
                }
                i++;
            }
        }
        //如果字符栈不为空，则开始最后的计算，最后数字栈里只有一个最后计算的结果
        while (!char_stack.isEmpty()){
            fetchAndCol(data_stack,char_stack);
        }
        return data_stack.pop();
    }

    //计算数字栈和运算符栈
    public void fetchAndCol(Stack<Integer> data_stack,Stack<Character> char_stack){
        Integer pop1 = data_stack.pop();
        Integer pop2 = data_stack.pop();
        Character pop = char_stack.pop();
        Integer math = math(pop1, pop2, pop);
        data_stack.push(math);
    }

    public Integer math(Integer number1,Integer number2,char c){
        if (c == '+') return number1 + number2;
        if (c == '-') return number2 - number1;
        if (c == '*') return number1 * number2;
        if (c == '/') return number2 / number1;
        else return -1;
    }

    public boolean checkChar(char c,char top){
        if ((c == '*' || c == '/') && (top == '+' || top == '-')) return true;
        if (top == '(') return true; //栈顶运算符是'(' 也是返回true
        else return false;
    }

    //判断是否是数字
    public boolean checkNumber(char c){
        if (c >= '0' && c <= '9') return true;
        else return false;
    }
}