150. Evaluate Reverse Polish Notation
Evaluate the value of an arithmetic expression in Reverse Polish Notation.
Valid operators are +, -, *, /. Each operand may be an integer or another expression.
使用栈，遇到操作数则入栈，操作符则出栈计算再把结果入栈，最后栈顶元素即为表达式的值。
from typing import List
class Solution:
_ops = ('+', '-', '*', '/')
def _calculate(self, op, num1, num2):
if op == '+':
return num1 + num2
elif op == '-':
return num1 - num2
elif op == '*':
return num1 * num2
else:
return int(num1 / num2)
def evalRPN(self, tokens: List[str]) -> int:
stack = []
for t in tokens:
if t not in self._ops:
stack.append(int(t))
else:
b = stack.pop()
a = stack.pop()
res = self._calculate(t, a, b)
stack.append(res)
return stack.pop()