题解 | #四则运算#

java代码解法，简单记录，方法比较笨，后续有机会继续改进。

import java.util.ArrayList;
public class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        String str = sc.nextLine();
        System.out.println(cal(trans(strToExp(str))));
    }
    //判断当前字符串是否是数字，包括负数
    public static boolean isNum(String str){
        for(int i=0;i<str.length();i++){
            if(i==0 && str.charAt(i) == '-' && str.length()>1){
                continue;
            }
            if(!Character.isDigit(str.charAt(i))){
                return false;
            }
        }
        return true;
    }
    //获取符号优先级，用作比较
    public static int getLv(String str){
        if("(".equals(str)) return 0;
        else if("-".equals(str)) return 1;
        else if("+".equals(str)) return 1;
        else if("/".equals(str)) return 2;
        else if("*".equals(str)) return 2;
        else return -1;
    }
    //计算后缀表达式
    public static int cal(List<String> res){
        if(res.size() == 1){
            return Integer.parseInt(res.get(1));
        }
        List<String> stack = new ArrayList<>();
        int top = -1;
        for(int i=0;i<res.size();i++){
            String str = res.get(i); 
            if(isNum(str)){
                stack.add(str);
                top++;
            }
            else {
                int x2 = Integer.parseInt(stack.get(top));
                int x1 = Integer.parseInt(stack.get(top-1));
                if("+".equals(str)){
                    Integer ans = x1+x2;
                    stack.remove(top);
                    stack.set(--top,ans.toString());
                }
                else if("-".equals(str)){
                    Integer ans = x1-x2;
                    stack.remove(top);
                    stack.set(--top,ans.toString());
                }
                else if("*".equals(str)){
                    Integer ans = x1*x2;
                    stack.remove(top);
                    stack.set(--top,ans.toString());
                }
                else if("/".equals(str)){
                    Integer ans = x1/x2;
                    stack.remove(top);
                    stack.set(--top,ans.toString());
                }
            }
        }
        return Integer.parseInt(stack.get(top));
    }
    
    //把拆分好的字符串数组改为后缀表达式
    public static List<String> trans(List<String> arr){
        List<String> res = new ArrayList<>();
        List<String> stack = new ArrayList<>();
        int top = -1;
        for(int i=0;i<arr.size();i++){
            String str = arr.get(i);
            if(isNum(str)){
                res.add(str);
            }
            else if(top == -1){
                stack.add(str);
                top++;
            }
            else if("(".equals(str) || "[".equals(str) || "{".equals(str)){
                stack.add(str);
                top++;
            }
            else if(")".equals(str)){
                while(!"(".equals(stack.get(top))){
                    res.add(stack.get(top));
                    stack.remove(top);
                    top--;
                }
                stack.remove(top);
                top--;
            }
            else if("]".equals(str)){
                while(!"[".equals(stack.get(top))){
                    res.add(stack.get(top));
                    stack.remove(top);
                    top--;
                }
                stack.remove(top);
                top--;
            }
            else if("}".equals(str)){
                while(!"{".equals(stack.get(top))){
                    res.add(stack.get(top));
                    stack.remove(top);
                    top--;
                }
                stack.remove(top);
                top--;
            }
            else if(getLv(str)<=getLv(stack.get(top))){
                res.add(stack.get(top));
                stack.remove(top--);
                while(top>-1 && getLv(str)<=getLv(stack.get(top))){
                    res.add(stack.get(top));
                    stack.remove(top--);
                }
                top++;
                stack.add(str);
            }
            else if(getLv(str)>getLv(stack.get(top))){
                stack.add(str);
                top++;
            }
        }
        while(top!=-1){
            res.add(stack.get(top));
            top--;
        }
        return res;
    }
    //将一个完整字符串拆分为一个字符串数组
    public static List<String> strToExp(String str){
        int len = str.length();
        StringBuffer sb = new StringBuffer();
        List<String> arr = new ArrayList<>();
        Character ch;
        for(int i=0;i<len;i++){
            ch = str.charAt(i);
            //考虑负号的情况
            if((i==0 && len>1 && ch=='-') || (i>0 && ch=='-' && (str.charAt(i-1)=='(' || str.charAt(i-1)=='[' || str.charAt(i-1)=='{'))){
                sb.append(ch);
            }
            //将数字合并
            else if(Character.isDigit(ch)){
                sb.append(ch);
                if((i+1<len && !Character.isDigit(str.charAt(i+1))) || (i+1==len)){
                    arr.add(sb.toString());
                    sb.setLength(0);
                }
            }
            //考虑其他加减乘除括号的情况
            else{
                sb.append(ch);
                arr.add(sb.toString());
                sb.setLength(0);
            }
        }
        return arr;
    }
}