Tutorial : STL Stack timer 1s memory 512MB

Let’s re-implement Beginner Coder problem #1102 using the STL stack.

The stack data structure can be implemented using the STL, and in practice, this is much more convenient.

Declaring a Stack

You can declare a stack as follows:

#include <stack>
using namespace std;

stack<int> s;

In the code above, <int> indicates the data type stored in the stack, and s is the name of the stack.
(The stack name can be chosen freely.)

If you declare it like this:

stack<char> s2;

then s2 becomes a stack that stores char values.

Commonly Used Stack Methods

After declaring a stack, the main methods you will use are the following:

Function 1: empty()

Usage example:

if (s.empty() == true) ...

The empty() function is a boolean function that returns true or false.

• Returns true if the stack is empty

• Returns false if the stack is not empty

Function 2: push()

Usage example:

s.push(7);

This inserts 7 at the top of the stack s.
In other words, 7 becomes the top element of s.

int k = 7;
s.push(k);

This also inserts 7 at the top of the stack.

Function 3: pop()

Usage example:

s.pop();

This removes the element at the top of the stack.

It only removes the element and does not return its value.

If pop() is called when the stack is empty, a runtime error occurs.

Therefore, always make it a habit to call empty() first to check whether the stack is empty before calling pop().

Function 4: top()

Usage example:

if (s.top() == 7) ...
// or
int t = s.top();

The top() function returns the value of the element at the top of the stack.

It only tells you the value and does not remove the element.

Just like pop(), calling top() on an empty stack causes a runtime error.

So always check whether the stack is empty using empty() before calling top().

Function 5: size()

Usage example:

if (s.size() == 0) ...
// or
int sz = s.size();

The size() function returns the number of elements in the stack.

If the stack is empty, size() returns 0.

Re-implementing Problem #1102

Now, using the five functions above, re-implement problem #1102.

< Processing Rules >

The given commands are as follows:

1. "i a": Insert the integer a into the stack.
   (a is a natural number less than or equal to 10,000.)

2. "o": Remove the top element from the stack and output it.
   If the stack is empty, output "empty".

3. "c": Output the number of elements currently stored in the stack.