Medium, Stack
Question
計(jì)算Reverse Polish Notation數(shù)值表達(dá)式的值. 有效的數(shù)值運(yùn)算符包括 +, -, *, /.
Examples:
["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9;
["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6
Notes
不會(huì)有空序列
Answer
利用stack LIFO的特點(diǎn)倔幼，每當(dāng)發(fā)現(xiàn)operater 的時(shí)候處理之前的兩個(gè)數(shù)潜腻，然后把處理結(jié)果推回堆棧须揣。
class Solution(object):
def evalRPN(self, tokens):
"""
:type tokens: List[str]
:rtype: int
"""
operators = {
'+': lambda x, y: x+y,
'-': lambda x, y: x-y,
'*': lambda x, y: x*y,
'/': lambda x, y: int(float(x)/ y )
}
stack = []
for token in tokens:
if token in operators:
y, x = stack.pop(), stack.pop()
stack.append(operators[token](x,y))
else:
stack.append(int(token))
return stack.pop()
