Expression Evaluation 数据流单调栈四则运算

LintCode 367,368
题目：
Given an expression string array, return the final result of this expression
Example 1:
For the expression 2*6-(23+7)/(1+2),
Input:
["2", "", "6", "-", "(","23", "+", "7", ")", "/", "(", "1", "+", "2", ")"]
Output:
2
Example 2:
For the expression 4-(2-3)*2+5/5,
Input:
["4", "-", "(", "2","-", "3", ")", "", "2", "+", "5", "/", "5"]
Output:
7
Notice
The expression contains only integer, +, -, *, /, (, ).

import java.util.*;

public class Solution {
    public static void main(String[] args) {
        String[] express = "2 * 6 - ( 23 + 7 ) / ( 1 + 2 )".split(" ");
        System.out.println(expressionEvaluation(express));
        String[] express2 = "3 + 6 / 2".split(" ");
        System.out.println(expressionEvaluation(express2));
    }
    public static int expressionEvaluation(String[] express){
        Map<String, Integer> map = new HashMap<>();
        map.put("+", 1);
        map.put("-", 1);
        map.put("*", 2);
        map.put("/", 2);
        int priority = 0;
        Deque<Operation> op_stack = new LinkedList<>();
        Deque<Integer> num_stack = new LinkedList<>();
        String str;
        for (int i=0; i<=express.length; i++){
            if (i<express.length){
                str = express[i];
            }else{
                str = "-";
            }
            if (map.keySet().contains(str)){
                Operation operation = new Operation(str, priority+map.get(str));
                while (!op_stack.isEmpty() && operation.priority<=op_stack.peekLast().priority){
                    String curOperation = op_stack.pollLast().op;
                    Integer curRes = null;
                    int b = num_stack.pollLast();
                    int a = num_stack.pollLast();
                    if (curOperation.equals("+")){
                        curRes = a+b;
                    }else if (curOperation.equals("-")){
                        curRes = a-b;
                    }else if (curOperation.equals("*")){
                        curRes = a*b;
                    }else if (curOperation.equals("/")){
                        curRes = a/b;
                    }
                    num_stack.offerLast(curRes);
                }
                op_stack.offerLast(operation);
            }else if (str.equals("(")){
                priority += 10;
            }else if (str.equals(")")){
                priority -= 10;
            }else {
                int num = Integer.parseInt(str);
                num_stack.offerLast(num);
            }
        }
        return num_stack.peekLast();
    }

    static class Operation{
        String op;
        int priority;
        Operation(String op, int priority){
            this.op = op;
            this.priority = priority;
        }
    }
}