Now I’m on to stacks. I had to rethink my strategy of going from right to left, to left to right. I was overcomplicating it.

---

Solution

class Solution:
    def evalRPN(self, tokens: List[str]) -> int:
        stack = []
        for token in tokens:
            if token in ['+', '/', '+', '-', '*']:
                right = int(stack.pop())
                left = int(stack.pop())
                match token:
                    case '+':
                        res = left + right
                    case '/':
                        res = left / right
                    case '+':
                        res = left + right
                    case '-':
                        res = left - right
                    case '*':
                        res = left * right
                stack.append(res)
            else:
                stack.append(token)
        return int(stack[-1])

            

O(n) = n since we iterate over all elements once. Space complexity also O(n) = n for the stack.