题目：
Evaluate the value of an arithmetic expression in Reverse Polish Notation.
Valid operators are +, -, *, /. Each operand may be an integer or another expression.
Some examples:
["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6
后缀表达式，用栈做，遇到运算符出栈两个元素做运算，然后把结果入栈。switch（“字符串”）是jdk1.7的新特性。
public class Solution {
public int evalRPN(String[] tokens) {
int temp = 0;
MyStack ms = new MyStack();
for(int i=0; i<tokens.length; i++){
switch(tokens[i]){
case "+": temp = ms.pop() + ms.pop(); ms.push(temp);break;
case "-": temp = -ms.pop() + ms.pop(); ms.push(temp);break;
case "*": temp = ms.pop() * ms.pop(); ms.push(temp);break;
case "/": temp = ms.pop();temp = ms.pop() / temp; ms.push(temp);break;
default: temp = Integer.parseInt(tokens[i]); ms.push(temp);
}
}
return ms.pop();
}
}
class MyStack{
int[] stack = new int[1000000];
int top = 1;
int pop(){
if(top == 0) return 0;
top--;
return stack[top];
}
void push(int a){
if(top > stack.length)
return;
stack[top] = a;
top++;
}
}