Question

Assume that the stack s contains the sequence of values 302544 , and that 44 is at the top. Write the values contained in s after each of the following methods are run, in sequence: a. \(s \cdot \operatorname{push}(55)\); b. s.pop(); C. s.pop();

Short answer

The stack is 30, 25 from bottom to top.

Step-by-step solution

Initial Stack

The initial stack sequence is 30, 25, 44 from bottom to top.

Execute s.push(55)

When the push method is called with the value 55, it adds 55 to the top of the stack. Now the stack sequence is 30, 25, 44, 55 from bottom to top.

Execute s.pop()

The pop method removes the top value from the stack. Removing 55 from the top leaves us with 30, 25, 44 from bottom to top.

Execute s.pop() again

Another pop operation removes the new top value, which is 44. Now the stack is left with 30, 25 from bottom to top.

Key concepts

Java push method

The `push` method in Java is used to add an element to the top of a stack. Imagine a stack of plates; when you add a new plate, you place it on top of the current plates. This is exactly how the `push` method works.
For instance, if our stack is represented as `[30, 25, 44]` with 44 on top, and we call the `push` method with the value `55` (i.e., `s.push(55)`), `55` will be placed on the top of the stack, making it `[30, 25, 44, 55]`.
Understanding the `push` method:

• It adds a new element at the top of the stack.
• The stack grows each time an element is pushed.
• It modifies the stack in-place, meaning the original stack is changed. No new stack is created.

By calling the `push` method, you can dynamically manage data as your program operates, adding new elements when needed.

Java pop method

The `pop` method in Java is used to remove the top element from a stack. Think of it like taking the topmost plate off from a stack of plates.
For instance, if we start with the stack `[30, 25, 44, 55]` with 55 at the top, calling `s.pop()` will remove `55`, leaving `[30, 25, 44]`.
Understanding the `pop` method:

• It removes the top element from the stack.
• The stack shrinks each time an element is popped.
• The removed element is usually returned so that it can be used if needed.

The `pop` method helps in managing data by easily removing unwanted or already processed elements, which is especially useful in algorithms and applications that rely on the Last-In-First-Out (LIFO) principle.

stack sequence

The sequence in a stack refers to the order in which elements are arranged from bottom to top. This is crucial when performing operations like `push` and `pop`. Let’s revisit our example:
1. Initial Stack Sequence: `[30, 25, 44]` (44 is at the top)
2. After `s.push(55)`: The stack becomes `[30, 25, 44, 55]` (55 is now at the top)
3. After first `s.pop()`: The stack goes back to `[30, 25, 44]` (44 returns to the top)
4. After second `s.pop()`: Finally, the stack is `[30, 25]` (25 is at the top now)
Understanding stack sequence helps you visualize and predict how the stack will look after a series of operations. This is particularly useful when debugging code or developing complex algorithms.

• Initial sequence is crucial, as all operations impact it.
• Tracking the top element is essential.
• The sequence evolves dynamically as `push` and `pop` methods are called.

Keeping the stack sequence in mind aids in better comprehension and effective programming when working with stack data structures.