求问以下的前缀表达式代码应该怎么优化

Question

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Stack;

public class PrefixExpression5 {
	
	// store some constant values
	final static long num = 1000000007;
	final static long max = Long.MAX_VALUE;
	
	public static void main(String[] args) throws IOException {
		
		// construct a stack to store the calculate values
		Stack<Long> st = new Stack<>();
		// get the array that stores the input data
		String[] elements = readData();
		// calculate the result
        long result = calculate(elements, st);
        // the prefix expression is invalid
        if (result == max) {
        	System.out.println("Invalid");
        }
        // the prefix expression is valid
        else {
        	// handle the result
        	result = (result + num) % num;
        	System.out.println(result);
        }
    }
    
	public static String[] readData() throws IOException {
		// construct a BufferedReader instance
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        // real the first line
        // split the first line
        // read n and k respectively
        String line = br.readLine();
        int n = Integer.parseInt(line);
        //System.out.println(n);
		String[] elements = new String[n];
		// read the data
        for (int i = 0; i < n; i++) {
        	elements[i] = br.readLine();
        }
        br.close();
        return elements;
	}
	
	public static long calculate(String[] arr, Stack<Long> st) {
		
		// store some values that will be used later
		String plus = "+";
		String minus = "-";
		long calRes;
		
		for (int i = arr.length - 1; i >= 0; i--) {
			String temp = arr[i];
			// if temp is numeric
			if (temp.toString().matches("^[0-9]*$")) {
				// push the numeric value to the stack
				st.push(Long.parseLong(arr[i]) % num);
			}
			else {
				// the expression is not correct
				if (st.size() < 2) {
					return max;
				}
				// pop two numbers from the stack and calculate
				long a = st.pop();
				long b = st.pop();
				if (temp.equals(plus)) {
					calRes = (a + b) % num;
				}
				else if (temp.equals(minus)) {
					calRes = ((a - b) % num);
				}
				else {
					calRes = ((a * b) % num);
				}
				// push the result into the stack
				st.push(calRes);
				}
			}
		// return the result
		return st.pop();
	}
}

输入样例

5
*
10
+
20
30

输出

500

 

oj测试最后一个点超过1s限制没过

 

求问应如何优化

Answer 1

使用双端队列Deque代替Stack，应该会加快你代码的效率，使用数组效率可能会更高，但要看你怎么写了，代码可能会变的很恶心，建议Deque代替Stack