Evaluate the value of an arithmetic expression in Reverse Polish Notation.
Valid operators are +, -, *, /. Each operand may be an integer or another expression.
Note:
       Division between two integers should truncate toward zero.
       The given RPN expression is always valid. That means the expression would always evaluate to a result and there won't be any divide by zero operation.
Example 1:
Input: ["2", "1", "+", "3", "*"]
Output: 9
Explanation: ((2 + 1) * 3) = 9
Example 2:
Input: ["4", "13", "5", "/", "+"]
Output: 6
Explanation: (4 + (13 / 5)) = 6
Example 3:
Input: ["10", "6", "9", "3", "+", "-11", "*", "/", "*", "17", "+", "5", "+"]
Output: 22
Explanation: 
  ((10 * (6 / ((9 + 3) * -11))) + 17) + 5
= ((10 * (6 / (12 * -11))) + 17) + 5
= ((10 * (6 / -132)) + 17) + 5
= ((10 * 0) + 17) + 5
= (0 + 17) + 5
= 17 + 5
= 22
class Solution:
   def evalRPN(self, tokens):
     """
     :type tokens: List[str]
     :rtype: int
     """
     s = []
     for token in tokens:
       if token == "+":
         a = int(s.pop())
         b = int(s.pop())
         s.append(a+b)
       elif token == "/":
         a = int(s.pop())
         b = int(s.pop())
         s.append(b/a)
       elif token == "*":
         a = int(s.pop())
         b = int(s.pop())
         s.append(a*b)
       elif token == "-":
         a = int(s.pop())
         b = int(s.pop())
         s.append(b-a)
       else:
         s.append(token)
     if len(s) is not 1:
       return False
     else:
       return int(s.pop())
class Solution:
   def evalRPN(self, tokens):
     """
     :type tokens: List[str]
     :rtype: int
     """
     s = []
     for token in tokens:
       if token == "+":
         a = int(s.pop())
         b = int(s.pop())
         s.append(a+b)
       elif token == "/":
         a = int(s.pop())
         b = int(s.pop())
         s.append(b/a)
       elif token == "*":
         a = int(s.pop())
         b = int(s.pop())
         s.append(a*b)
       elif token == "-":
         a = int(s.pop())
         b = int(s.pop())
         s.append(b-a)
       else:
         s.append(token)
     if len(s) is not 1:
       return False
     else:
       return int(s.pop())