题解 | #表达式求值# 中缀 转 后缀，再计算后缀

/**
很传统的处理方式，但是我感觉考试的时候根本就写不完
*/
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;
import java.util.Stack;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    private static int showPrio(Character c){
        if(c.equals('*') || (c.equals('/'))){
            return 1;
        }else if(c.equals('+') || (c.equals('-'))) {
            return 0;
        }
        return -1;
    }
    private static boolean ifPop(Stack<Character> s,Character c){
        int a=0,b=0;
        if(s.empty()) return false;
        a=showPrio(s.peek());
        b=showPrio(c);
        if(a==-1||b==-1) return false;
        return a>=b;
    }
    private static List<String> conv2postfix(String str) {
        char[] s = str.trim().toCharArray();
        Stack<Character> stack = new Stack<>();
        Character c;
        int num = 0;
        List<String> list = new ArrayList<>();
        for (int i = 0; i < s.length; i++) {
            if (Character.isDigit(s[i])) {
//                System.out.print(s[i]);
                num = num * 10 + s[i] - '0';
                if (i + 1 >= s.length || !Character.isDigit(s[i + 1])) {
                    list.add(String.valueOf(num));
                    num = 0;
                }
            } else {
                if (s[i] == ')') {
                    while ((c = stack.pop()) != '(') {
//                        System.out.print(c);
                        list.add(String.valueOf(c));
                    }
                    continue;
                }
                //依次将栈中比它优先级高或等于它的操作符弹出并输出，然后将该操作符压入栈中。
                while (ifPop(stack, s[i])) {
//                    System.out.print(stack.pop());
                    list.add(String.valueOf(stack.pop()));
                }
                stack.push(s[i]);
            }
        }
        while (!stack.empty()) {
//            System.out.print(stack.pop());
            list.add(String.valueOf(stack.pop()));
        }
        return list;
    }

    private static String calpostfix(List<String> list) {
        Stack<String> stack = new Stack<>();
        list.stream().forEach(i-> {
            if (i.equals("/") || i.equals("*") || i.equals("+") || i.equals("-")) {
                int a = 0, b = 0;
                if (stack.size() >= 2) {
                    b = Integer.valueOf(stack.pop());
                    a = Integer.valueOf(stack.pop());
                }
                switch (i) {
                    case "/":
                        stack.push(String.valueOf(a / b));
                        break;
                    case "*":
                        stack.push(String.valueOf(a * b));
                        break;
                    case "+":
                        stack.push(String.valueOf(a + b));
                        break;
                    case "-":
                        stack.push(String.valueOf(a - b));
                        break;
                }
            } else {
                stack.push(i);
            }
        });
        return stack.pop();
    }

    public static void main(String args[]) {
        Scanner sc = new Scanner(System.in);
        StringBuffer sb=new StringBuffer(sc.nextLine());
	  //把一元运算符（正负号） 转化为 两个数做运算
        for(int i=0;i<sb.length();i++){
            if(sb.charAt(i)=='-' || sb.charAt(i)=='+'){
                if(i-1<0 || sb.charAt(i-1)=='('){
                    sb.insert(i,'0');
                }
            }
        }
        List<String> list = conv2postfix(sb.toString());
        //list.stream().forEach(i->System.out.print(i+" "));
        //System.out.println("");
        System.out.println(calpostfix(list));
    }
}