150. Evaluate Reverse Polish Notation【M】【19】

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

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class Solution(object):
def evalRPN(self, tokens):

s = []

for i in tokens:

if i == "+":
t = s[-1] + s[-2]
s = s[:-2]
s += t,
elif i == "-":
t = s[-2] - s[-1]
s = s[:-2]
s += t,
elif i == "*":
t = s[-2] * s[-1]
s = s[:-2]
s += t,
elif i == "/":
if s[-2]*s[-1] < 0:
t = - (abs(s[-2]) / abs(s[-1]))
else:
t = s[-2] / s[-1]
s = s[:-2]
s += t,
else:
i = int(i)
s += i,
#print s
return s[0]