Java 五則運算

題目:五則運算

參考 中序式轉後序式 後序式運算

注意:此程式用Java SE7去寫，如果是用SE6以下去編譯，switch部分會編譯錯誤

import java.util.*;

public class FiveOperations{
    public static int priority(String str){
        switch( str ){
            case "+": case "-": 
                return 1;
            case "*": case"/": case "%": 
                return 2;
            default: 
                return 0;
        }
    }
 
    public static int count(String op, int a, int b){
        switch( op ){
            case "+": return a + b;
            case "-": return a - b;
            case "*": return a * b;
            case "/": return a / b;
            case "%": return a % b;
            default: throw new ArithmeticException(op + " not defined");
        }
    }

    public static String toPostfix(String str){
        StringBuffer sb = new StringBuffer();
        LinkedList<String> stack = new LinkedList<String>();
        for(String sub : str.split(" ") ){
            if("(".indexOf(sub) != -1){
                stack.add(sub);
            }else if("+-*/%".indexOf(sub) != -1){
                while(!stack.isEmpty() && priority(stack.getLast()) >= priority(sub)){
                    sb.append(stack.removeLast() + " ");
                }
                stack.add(sub);
            }else if(")".indexOf(sub) != -1){
                while( "(".indexOf(stack.getLast()) == -1 ){
                    sb.append(stack.removeLast() + " ");
                }
                stack.removeLast();
            }else{
                sb.append(sub + " ");
            }
        }
  
        while(!stack.isEmpty()){ sb.append(stack.removeLast() + " "); }
        return sb.toString();
    }
 
    public static int answer(String str){
        Stack<Integer> stack = new Stack<Integer>();
        for(String sub : toPostfix(str).split(" ")){
            if("+-*/%".indexOf(sub) != -1){
                int b = stack.pop();
                int a = stack.pop();
                stack.push(count(sub, a, b));
            }else{
                stack.push(Integer.parseInt(String.valueOf(sub)));
            }
        }
        return stack.pop();
    }
 
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        String str;
  
        while( sc.hasNext() ){
            str = sc.nextLine();
            System.out.println( answer(str) );
        }
    }
}