逆波兰计算器

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: