evaluate the value of arithmetic expression in reverse polish notation. Valid operator are ,-,*,/. Each operand may be an integer or another expression. Some examples: [2, 1, , 3, *] - ((21)*3) - 9 [4, 13, 5, /, ] - (4 (13 / 5)) - 6 分析：
evaluate the value of arithmetic expression in reverse polish notation.
Valid operator 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
分析：后缀表达式操作。
栈的应用，如果碰见数字，则压栈，碰见运算符则弹出两个元素，对两个元素进行数学运算后结果压栈。
public class Solution {
public int evalRPN(String[] tokens) {
Stack st = new Stack();
for(String token : tokens){
if(token.matches("-?[0-9]+")){
st.push(Integer.parseInt(token));
}else{
int num2 = st.pop();
int num1 = st.pop();
if(token.equals("+")){
st.push(num1+num2);
}else if(token.equals("-")){
st.push(num1-num2);
}else if(token.equals("*")){
st.push(num1*num2);
}else if(token.equals("/")){
st.push(num1/num2);
}
}
}
return st.pop();
}
} 
