Exercise 1 solution

    def rpn(expr):
        st = []
        for tok in expr.split():
            if tok in ("+", "-", "*"):
                b = st.pop()      # right operand comes off first
                a = st.pop()
                if tok == "+":
                    st.append(a + b)
                elif tok == "-":
                    st.append(a - b)
                else:
                    st.append(a * b)
            else:
                st.append(int(tok))
        return st[0]

    print(rpn("3 4 + 2 *"), rpn("5 1 2 + 4 * + 3 -"))

Output:
    14 14

The first value popped is the right-hand operand, so for "-" the order
a - b matters. Swapping them would give wrong answers for subtraction.