AN OPERATOR ALWAYS ACTS ON THE TWO MOST RECENT NUMBERS. A STACK REMEMBERS THEM IN ORDER: PUSH EACH NUMBER, AND WHEN AN OPERATOR ARRIVES POP THE TOP TWO, COMBINE THEM, AND PUSH THE RESULT BACK. THE LAST VALUE LEFT IS THE ANSWER.

TOKENS SCAN LEFT TO RIGHT. A NUMBER PUSHES ONTO THE STACK. AN OPERATOR POPS THE TOP TWO, LEFT IN BLUE AND RIGHT IN PINK, COMBINES THEM, AND PUSHES THE RESULT IN GREEN. WATCH 13 / 5 TRUNCATE TOWARD ZERO TO 2.

1def eval_rpn(tokens):2    stack = []3    ops = {4        "+": lambda a, b: a + b,5        "-": lambda a, b: a - b,6        "*": lambda a, b: a * b,7        "/": lambda a, b: int(a / b)8    }9    for t in tokens:10        if t in ops:11            right = stack.pop()12            left = stack.pop()13            stack.append(ops[t](left, right))14        else:15            stack.append(int(t))16    return stack[0]