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.


Example 1:


Input: ["2", "1", "+", "3", "*"]
Output: 9
Explanation: ((2 + 1) * 3) = 9


Example 2:


Input: ["4", "13", "5", "/", "+"]
Output: 6
Explanation: (4 + (13 / 5)) = 6


Example 3:


Input: ["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

Source: leetcode | Difficulty: MEDIUM_HARD
Tags: Data structures

Mathematics

Starter code: class Solution:
    def evalRPN(self, tokens: List[str]) -> int:
        

Solutions:
class Solution:

	def evalRPN(self, tokens):
		s = []
		for token in tokens:
			if token == '+':
				a = int(s.pop())
				b = int(s.pop())
				s.append(a + b)
			elif token == '/':
				a = int(s.pop())
				b = int(s.pop())
				s.append(b / a)
			elif token == '*':
				a = int(s.pop())
				b = int(s.pop())
				s.append(a * b)
			elif token == '-':
				a = int(s.pop())
				b = int(s.pop())
				s.append(b - a)
			else:
				s.append(token)
		if len(s) is not 1:
			return False
		else:
			return int(s.pop())