leetcode-150题:evaluate reverse polish notation

def evalRPN(self, tokens):
    """
    :type tokens: List[str]
    :rtype: int
    """
    if tokens==None or len(tokens)==0:
        return 0
    num_stack = []
    for c in tokens:
        if c == '+':
            b = num_stack.pop()
            a = num_stack.pop()
            num_stack.append(int(a)+int(b))
        elif c == '-':
            b = num_stack.pop()
            a = num_stack.pop()
            num_stack.append(int(a)-int(b))
        elif c == '*':
            b = num_stack.pop()
            a = num_stack.pop()
            num_stack.append(int(a)*int(b))
        elif c == '/':
            b = num_stack.pop()
            a = num_stack.pop()
            if a/b < 0 and a%b!=0:
                num_stack.append(a/b+1)
            else:
                num_stack.append(a/b)
        else:
            num_stack.append(int(c))
    return num_stack[0]