150. evaluate reverse polish notation

class Solution(object):
    def evalRPN(self, tokens):
        """
        :type tokens: List[str]
        :rtype: int
        """
        queue = []
        for item in tokens:
            if item == "*" or item == "/" or item == "+" or item == "-":
                operand2 = queue.pop()
                operand1 = queue.pop()
                result = self.calc(item, operand1, operand2)
                queue.append(result)
            else:
                queue.append(int(item))
    <span class="k">return</span> <span class="n">queue</span><span class="o">.</span><span class="n">pop</span><span class="p">()</span>

<span class="k">def</span> <span class="nf">calc</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">op</span><span class="p">,</span> <span class="n">op1</span><span class="p">,</span> <span class="n">op2</span><span class="p">):</span>
    <span class="k">if</span> <span class="n">op</span> <span class="o">==</span> <span class="s">&#34;*&#34;</span><span class="p">:</span>
        <span class="k">return</span> <span class="n">op1</span> <span class="o">*</span> <span class="n">op2</span>
    <span class="k">elif</span> <span class="n">op</span> <span class="o">==</span> <span class="s">&#34;/&#34;</span><span class="p">:</span>
        <span class="k">if</span> <span class="n">op1</span> <span class="o">*</span> <span class="n">op2</span> <span class="o">&lt;</span> <span class="mi">0</span><span class="p">:</span>    <span class="c"># python是向下取整，即5/(-3)=-2</span>
            <span class="k">return</span> <span class="o">-</span><span class="p">((</span><span class="o">-</span><span class="n">op1</span><span class="p">)</span> <span class="o">/</span> <span class="n">op2</span><span class="p">)</span>
        <span class="k">else</span><span class="p">:</span>
            <span class="k">return</span> <span class="n">op1</span> <span class="o">/</span> <span class="n">op2</span>
    <span class="k">elif</span> <span class="n">op</span> <span class="o">==</span> <span class="s">&#34;+&#34;</span><span class="p">:</span>
        <span class="k">return</span> <span class="n">op1</span> <span class="o">+</span> <span class="n">op2</span>
    <span class="k">else</span><span class="p">:</span>
        <span class="k">return</span> <span class="n">op1</span> <span class="o">-</span> <span class="n">op2</span>

if name == 'main':

answer = Solution()

print answer.evalRPN(["10","6","9","3","+","-11","","/","","17","+","5","+"])