题目： Evaluate the value of an arithmetic expression in Reverse Polish Notation.
Valid operators 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
思路：题目本身并没有难度，维护一个数字栈，每次遇到操作符便弹出栈顶的两个数字进行操作即可。可是Python涉及负数的除法规则和Java等不同，需注意，-1/２在Python中是-1而在Java中是０。
代码：
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]
