Stack

A stack is a linear data structure because its elements are arranged in a sequence, and insertion/deletion happen from one end only.

A stack follows the LIFO principle, so the statement that it does not follow LIFO is incorrect.

PUSH is an insertion operation. Underflow happens when we try to POP from an empty stack, not when we push.

In postfix notation, the operators are written after the operands.

Start with `result = 0` and `numberList = [10,20,30]`.

- `numberList.append(40)` makes the list `[10,20,30,40]`
- First `pop()` gives `40`, so `result = 0 + 40 = 40`
- Second `pop()` gives `30`, so `result = 40 + 30 = 70`

So the output is `Result= 70`.

The list acts as a stack:

- `answer.append('T')` → `['T']`
- `answer.append('A')` → `['T','A']`
- `answer.append('M')` → `['T','A','M']`

Now popping removes from the top:

- `pop()` gives `M` → `output = "M"`
- `pop()` gives `A` → `output = "MA"`
- `pop()` gives `T` → `output = "MAT"`

So the printed result is `Result= MAT`.