150. Evaluate Reverse Polish Notation

문제)

150. Evaluate Reverse Polish Notation

 

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 <= 104
• tokens[i] is either an operator: "+", "-", "*", or "/", or an integer in the range [-200, 200].

문제 풀이)

- 스택 자료구조 사용

- 토큰을 순회하면서 숫자면 스택에 푸시, 아니면 스택에서 pop 두번해서 연상 결과를 다시 스택에 푸시

솔루션1)

class Solution:
    def evalRPN(self, tokens: List[str]) -> int:
        operators = {'+', '-', '*', '/'}

        # use stack
        stack = []

        for token in tokens:
            if token in operators:
                a = stack.pop()
                b = stack.pop()
                if token == '+':
                    c = a + b
                elif token == '-':
                    c = b - a
                elif token == '*':
                    c = a * b
                else:                    
                    c = int(b / a)
                stack.append(c)
            else:
                stack.append(int(token))

        return stack[0]