Stack State Management

Use a stack when your algorithm has explicit phases or scopes (branches, DFS frames, undo logs). Push on enter, pop on exit.

Try this in Python

def eval_rpn(tokens: list[str]) -> int:
    st: list[int] = []
    for t in tokens:
        if t in "+-*":
            b = st.pop()
            a = st.pop()
            if t == "+":
                st.append(a + b)
            elif t == "-":
                st.append(a - b)
            else:
                st.append(a * b)
        else:
            st.append(int(t))
    return st[-1]


print(eval_rpn(["2", "1", "+", "3", "*"]))

Key takeaways

• RPN is a tiny language of deferred operations—classic stack state.
• Each operator collapses the top two states into one.
• Think “what is waiting for more operands?”