evaluate reverse polish notation

correct

class Solution:
    def evalRPN(self, tokens: List[str]) -> int:
        """
        ["1","2","+","3","*","4","-"]
        """
        numStack = []
        for t in tokens:
            if t in "+-*/":
                num1 = numStack.pop()
                num2 = numStack.pop()
                if t == "+":
                    val= num1 + num2
                elif t == "-":
                    val = num2 - num1
                elif t == "/":
                    val = int(float(num2) / num1)
                elif t =="*":
                    val = num1 * num2
                numStack.append(val) 
            else:
                numStack.append(int(t))
        
        return numStack[0]

• reverse the if else case (from the wrong answer below)
• be careful with the order of operations
  • left - right
  • left / right
• Assume that division between integers always truncates toward zero.
  • int(float(num2) / num1)
  • int(num2 / num1)
• O(n)
• one note:
  • it’s good to debug where it might have gone wrong, because i didn’t really know that my error was from t.isnumeric()

wrong!!

class Solution:
    def evalRPN(self, tokens: List[str]) -> int:
        """
        ["1","2","+","3","*","4","-"]
        """
        numStack = []
        for t in tokens:
            if t.isnumeric():
                numStack.append(int(t))
            else: # is an operand
                val = 0
                if (numStack):
                    num1 = numStack.pop()
                    num2 = numStack.pop()
                    if t == "+":
                        val = num1 + num2
                    elif t == "-":
                        val = num2 - num1
                    elif t == "/":
                        val = int(float(num2) / num1)
                    elif t =="*":
                        val = num1 * num2
                numStack.append(val)
        
        return numStack[0]

• my initial answer, but this doesnt work because t.isnumeric() doesnt evaluate correctly for negative numbers like -4