150. Evaluate Reverse Polish Notation

class Solution:
    def evalRPN(self, tokens: List[str]) -> int:
        stack = []
        
        for token in tokens:
            if token.isnumeric() or token[1:].isnumeric():
                stack.append(int(token))
            elif token == "+":
                val = stack.pop() + stack.pop()
                stack.append(val)
            elif token == "-":
                val = -(stack.pop() - stack.pop())
                stack.append(val)
            elif token == "/":
                val2 = stack.pop()
                val1 = stack.pop()
                stack.append(int(val1 / val2))
            elif token == "*":
                val = stack.pop() * stack.pop()
                stack.append(val)
        
        return stack[-1]

• we can evaluate from the left by using a stack.
• when we see an operator, we pop off the two most recent values in the stack, evaluate, and then push that new value onto the stack.
• note that we can do integer division truncating towards zero by doing int(a / b).

Categories:: array, math, stack