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[12/17] 150. Evaluate Reverse Polish Notation
본문
Evaluate the value of an arithmetic expression in Reverse Polish Notation.
Valid operators are +, -, *, and /. Each operand may be an integer or another expression.
Note that division between two integers should truncate toward zero.
It is guaranteed that the given RPN expression is always valid. That means the expression would always evaluate to a result, and there will not be any division by zero operation.
Example 1:
Input: tokens = ["2","1","+","3","*"] Output: 9 Explanation: ((2 + 1) * 3) = 9
Example 2:
Input: tokens = ["4","13","5","/","+"] Output: 6 Explanation: (4 + (13 / 5)) = 6
Example 3:
Input: tokens = ["10","6","9","3","+","-11","*","/","*","17","+","5","+"] Output: 22 Explanation: ((10 * (6 / ((9 + 3) * -11))) + 17) + 5 = ((10 * (6 / (12 * -11))) + 17) + 5 = ((10 * (6 / -132)) + 17) + 5 = ((10 * 0) + 17) + 5 = (0 + 17) + 5 = 17 + 5 = 22
Constraints:
1 <= tokens.length <= 104tokens[i]is either an operator: <code style="border: 1px solid rgba(247, 250, 255, 0.12); box-sizing: border-box; --tw-border-spacing-x:0; --tw-border-spacing-y:0; --tw-translate-x:0; --tw-translate-y:0; --tw-rotate:0; --tw-skew-x:0; --tw-skew-y:0; --tw-scale-x:1; --tw-scale-y:1; --tw-pan-x: ; --tw-pan-y: ; --tw-pinch-zoom: ; --tw-scroll-snap-strict
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class Solution:
def evalRPN(self, tokens: List[str]) -> int:
stack = []
operators = "/*+-"
def calc(op):
num2 = stack.pop()
num1 = stack.pop()
if op == '+':
stack.append(num1+num2)
elif op == '-':
stack.append(num1-num2)
elif op == '*':
stack.append(num1*num2)
elif op == '/':
stack.append(int(num1/num2))
for string in tokens:
if string[0].isdigit():
stack.append(int(string))
elif string in operators:
calc(string)
elif string[0] == "-":
stack.append(-int(string[1:]))
return stack[-1]
