奇怪的表达式求值--网易2017春招实习笔试编程题8

输入为一行字符串，即一个表达式。其中运算符只有-,+,*。参与计算的数字只有0~9.
保证表达式都是合法的，排列规则如样例所示。

输出一个数，即表达式的值

3+5*7

56

import java.util.Scanner;
import java.util.Stack;

public class Main {
public int calculate(String s) {
Stack<Integer> nums = new Stack<Integer>();
Stack<Character> operators = new Stack<Character>();
char[] c = s.toCharArray();
for (int i = c.length - 1; i >= 0; i--) {
if (c[i] == '+' || c[i] == '-' || c[i] == '*') {
operators.push(c[i]);
} else if (c[i] >= '0' && c[i] <= '9') {
nums.push(c[i] - '0');
}
}
Integer a = null;
Integer b = null;
Character operator = null;
while (!nums.empty() && !operators.empty()) {
a = nums.pop();
b = nums.pop();
operator = operators.pop();
if (operator == '+')
nums.push(a + b);
else if (operator == '-')
nums.push(a - b);
else if (operator == '*')
nums.push(a * b);
}
return nums.pop();//最后，数字栈中有一个表达式的结果值，操作符栈已空
}//calculate

public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
String s = sc.next();
System.out.println(new Main().calculate(s));
sc.close();
}//main
}