150. Evaluate Reverse Polish Notation

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

解题要点：

遍历list，把所有不是运算符的加进stack（只含数字）。当遇到运算符时，pop两个element出来，并进行运算，此时的顺序为stack底部的在前，顶部在运算符后。算好后把新数字再加进stack里，以此类推，直到最后返回stack剩下的那个数，为最终结果。

class Solution(object):
    def evalRPN(self, tokens):
        """
        :type tokens: List[str]
        :rtype: int
        """
        stNum = []
        if tokens == None or len(tokens) == 0:
            return 0
        
        for n in tokens:
            if n == "+" or n == "-" or n == "*" or n == "/":
                second = stNum.pop()
                first = stNum.pop()
                if n == "+":
                    stNum.append(first + second)
                elif n == "-":
                    stNum.append(first - second)
                elif n == "*":
                    stNum.append(first * second)
                elif n == "/":
                    stNum.append(int(operator.truediv(first, second)))
            else:
                stNum.append(int(n))
        return stNum.pop()