前缀表达式计算规则：
1. 从后往前读
2. 遇数字进栈，遇操作符栈顶两个出栈计算，结果再入栈。
这里假定前缀表达式是合法的，没有做合法性校验。
AC代码：
import java.util.Scanner;
import java.util.Stack;
/**
* @author CC11001100
*/
public class Main {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
while (sc.hasNextLine()) {
sc.nextLine();
System.out.println(prefixComplete(sc.nextLine()));
}
}
private static int prefixComplete(String expression) {
Stack<Integer> stack = new Stack<>();
String[] ss = expression.split("\s+");
for (int i = ss.length - 1; i >= 0; i--) {
String s = ss[i];
switch (s) {
case "+":
stack.push(stack.pop() + stack.pop());
break;
case "-":
stack.push(stack.pop() - stack.pop());
break;
case "*":
stack.push(stack.pop() * stack.pop());
break;
case "/":
stack.push(stack.pop() / stack.pop());
break;
default:
stack.push(Integer.valueOf(s));
}
}
return stack.pop();
}
}