逆波兰计算器

Posted 2022-05-17 *平芜尽处是春山*

逆波兰计算器

大体思路：将中缀表达式转为后缀表达式后再通过逆波兰计算器计算。
代码实现：

import java.util.ArrayList;
import java.util.List;
import java.util.Stack;

public class calculator 
    public static void main(String[] args) 
        String expression = "(1+4)*5-8";
        List<String> infixExpressionList = toInfixExpressionList(expression);
        System.out.println("中缀表达式对应的List=" + infixExpressionList); // ArrayList [(, 3, +, 4, ), *, 5, -, 6]
        List<String> suffixExpressionList = parseSuffixExpressionList(infixExpressionList);
        System.out.println("后缀表达式对应的List" + suffixExpressionList); //ArrayList [3, 4, +, 5, *, 6, -]
        System.out.printf("expression=%d", calculate(suffixExpressionList));
    
    
    public static List<String> toInfixExpressionList(String s) 
        List<String> ls = new ArrayList<String>();
        int i = 0;
        String str;
        char c;
        do 
            if((c=s.charAt(i)) < 48 ||  (c=s.charAt(i)) > 57) 
                ls.add("" + c);
                i++;
             else 
                str = "";
                while(i < s.length() && (c=s.charAt(i)) >= 48 && (c=s.charAt(i)) <= 57) 
                    str += c;
                    i++;
                
                ls.add(str);
            
        while(i < s.length());
        return ls;
    

    public static List<String> parseSuffixExpressionList(List<String> ls) 
        Stack<String> s1 = new Stack<String>();
        //Stack<String> s2 = new Stack<String>();
        List<String> s2 = new ArrayList<String>();
        for(String item: ls) 
            if(item.matches("\\\\d+")) 
                s2.add(item);
             else if (item.equals("(")) 
                s1.push(item);
             else if (item.equals(")")) 
                while(!s1.peek().equals("(")) 
                    s2.add(s1.pop());
                
                s1.pop();
             else 
                while(s1.size() != 0 && Operation.getValue(s1.peek()) >= Operation.getValue(item) ) 
                    s2.add(s1.pop());
                
                s1.push(item);
            
        
        while(s1.size() != 0) 
            s2.add(s1.pop());
        
        return s2;
    
    
    public static int calculate(List<String> ls) 
        Stack<String> stack = new Stack<String>();
        for (String item : ls) 
            if (item.matches("\\\\d+")) 
                stack.push(item);
             else 
                int num2 = Integer.parseInt(stack.pop());
                int num1 = Integer.parseInt(stack.pop());
                int res = 0;
                if (item.equals("+")) 
                    res = num1 + num2;
                 else if (item.equals("-")) 
                    res = num1 - num2;
                 else if (item.equals("*")) 
                    res = num1 * num2;
                 else if (item.equals("/")) 
                    res = num1 / num2;
                 else 
                    throw new RuntimeException("运算符有误");
                
                stack.push("" + res);
            
        
        return Integer.parseInt(stack.pop());
    


class Operation 
    private static int ADD = 1;
    private static int SUB = 1;
    private static int MUL = 2;
    private static int DIV = 2;

    public static int getValue(String operation) 
        int result = 0;
        switch (operation) 
            case "+":
                result = ADD;
                break;
            case "-":
                result = SUB;
                break;
            case "*":
                result = MUL;
                break;
            case "/":
                result = DIV;
                break;
            default:
                System.out.println("不存在该运算符" + operation);
                break;
        
        return result;
    

运行截图: