stack is an ADT (abstract data type ) that act like list of objects but there is a diffrents.

stack act is *LIFO* (Last In First Out), it means that when we want to get an element from the stack we get the last element in the stack.

stack is bast on two methods ( functions)

**push**: add an alement to last index of stack.

for example: we have `1, 3, 5` in stack, then we call push(9),

`9` will add to last index of stack -> `1, 3, 5 , 9`

**pop**: remove the last element from stack.

for example: we have `1, 3, 5 , 9` in stack, then we call pop(),

the function will return `9` and the stack will change to `1, 3, 5`.