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.

Note:

• Division between two integers should truncate toward zero.
• The given RPN expression is always valid. That means the expression would always evaluate to a result and there won't be any divide by zero operation.

My Answer 1: Output Limit Exceeded (18 / 20 test cases passed.)

class Solution:
    def evalRPN(self, tokens: List[str]) -> int:
        operator = ['+', '-', '*', '/']
        
        while len(tokens) != 1:
            a = 0
            b = 0
            for i in range(len(tokens)-2):
                if tokens[i+2] in operator:
                    a = int(tokens[i])
                    b = int(tokens[i+1])
                    print(i, a, b)
                    
                    if tokens[i+2] == operator[0]:
                        tokens[i+2] = str(a+b)
                    elif tokens[i+2] == operator[1]:
                        tokens[i+2] = str(a-b)
                    elif tokens[i+2] == operator[2]:
                        tokens[i+2] = str(a*b)
                    elif tokens[i+2] == operator[3]:
                        tokens[i+2] = str(int(a/b))
                    
                    print(tokens[i+2])
                    tokens.pop(i+1)
                    tokens.pop(i)
                    print(tokens)
                    break
        
        return tokens[0]

첨엔 tokens[i], tokens[i+1], tokens[i+2] 세개씩 세트로 보고
tokens[i+2] 가 연산자면 계산값을 넣고 i 와 i+1 은 지워주는 방식으로 했는데.. Output Limit Exceeded

My Answer 2: Accepted (Runtime: 60 ms - 93.21% / Memory Usage: 14.4 MB - 96.78%)

class Solution:
    def evalRPN(self, tokens: List[str]) -> int:
        operator = ['+', '-', '*', '/']
        stack = []
        
        for e in tokens:
            if e in operator:
                b = int(stack.pop())
                a = int(stack.pop())
                if e == operator[0]:
                    stack.append(a + b)
                elif e == operator[1]:
                    stack.append(a - b)
                elif e == operator[2]:
                    stack.append(a * b)
                else:
                    tmp = int(a/b)
                    stack.append(tmp)
            else:
                stack.append(e)
        return stack.pop()

stack 을 이용하면 된다..!!