Stack LIFO Principle

LIFO means the newest item leaves first. Any nested “undo” or “back” behavior maps naturally to a stack.

Try this in Python

def undo_stack(ops: list[str]) -> list[str]:
    st: list[str] = []
    for op in ops:
        if op == "back":
            if st:
                st.pop()
        else:
            st.append(op)
    return st


print(undo_stack(["open:a", "open:b", "back", "open:c"]))

Key takeaways

• Model reversible steps with push; model undo with guarded pop.
• Empty stack checks prevent crashes.
• Same idea powers DFS and recursion frames.