115 lines
2.9 KiB
Markdown
115 lines
2.9 KiB
Markdown
---
|
|
title: Stacks
|
|
---
|
|
## Stacks
|
|
|
|
A stack is a First In Last Out (FILO) or Last In First Out (LIFO) linear data structure.
|
|
|
|
Imagine the way plates are stacked in a buffet restaurant. You can only pick plates from top otherwise the stack will collapse.
|
|
|
|
Some basics operations of stack are:
|
|
1. Push() - Inserts the item on the top of the stack
|
|
2. Pop() - Removes the top item (often times, it is a good idea to implement this function so that it returns the element it removed)
|
|
3. isEmpty() - Check whether the stack is empty or not (returns a boolean)
|
|
4. Size() - Return the number of items in the stack
|
|
(All the operations can be done in O(1) time - depending on the implementation)
|
|
|
|
Implementation of a stack is possible using either arrays or linked lists. The following is a simple array implementation of the stack data structure with its most common operations.
|
|
|
|
```C++
|
|
//Stack implementation using array in C++
|
|
//You can also include<stack> and then use the C++ STL Library stack class.
|
|
|
|
#include <bits/stdc++.h>
|
|
|
|
using namespace std;
|
|
|
|
class Stack {
|
|
int t;
|
|
int arr[MaxN];
|
|
public:
|
|
Stack() {
|
|
t = 0;
|
|
}
|
|
int size() {
|
|
return t;
|
|
}
|
|
bool isEmpty() {
|
|
return t < 1;
|
|
}
|
|
int top() {
|
|
return arr[t];
|
|
}
|
|
void push(int x) {
|
|
if (++t >= MaxN) {
|
|
cout << "Stack is full" << '\n';
|
|
return;
|
|
}
|
|
arr[t] = x;
|
|
}
|
|
void pop() {
|
|
arr[t--] = 0;
|
|
}
|
|
};
|
|
|
|
int main() {
|
|
Stack st;
|
|
|
|
st.push(4);
|
|
st.push(3);
|
|
st.push(5);
|
|
while (!st.isEmpty()) {
|
|
cout << st.size() << ' ' << st.top() << '\n';
|
|
st.pop();
|
|
}
|
|
return 0;
|
|
}
|
|
```
|
|
|
|
#### Using Arrays as Stacks
|
|
|
|
In some programming languages an array has stack functionality, allowing the developer to perform **push** and **pop** operations without the need for a custom stack data structure.
|
|
|
|
For example, an array in JavaScript has **push** and **pop** methods allowing one to easily implement stack functionality in an application.
|
|
|
|
```js
|
|
stack = [];
|
|
|
|
let i = 0;
|
|
while(i < 5)
|
|
stack.push(i++);
|
|
|
|
while(stack.length) {
|
|
stack.pop();
|
|
}
|
|
```
|
|
|
|
A List in Python can also perform stack functionality in an application. Instead of **push**, one can use the **append** method.
|
|
```python
|
|
stack = []
|
|
|
|
for i in range(5):
|
|
stack.append(i)
|
|
|
|
while len(stack):
|
|
stack.pop()
|
|
```
|
|
|
|
#### Applications
|
|
|
|
* Turn recursion into loop.
|
|
* Redo-Undo features.
|
|
* Sudoku solver
|
|
* Depth First Search.
|
|
* Tree traversals
|
|
* Infix expression -> Prefix/Postfix expression
|
|
* Valid Parentheses
|
|
|
|
|
|
|
|
#### More Information:
|
|
* [More Info on Stacks - GeeksForGeeks](http://www.geeksforgeeks.org/stack-data-structure/)
|
|
* [Stack - Wikipedia](https://en.wikipedia.org/wiki/Stack_(abstract_data_type)
|
|
* [Tower of Hanoi Problem and how the solution uses stacks and recursions](https://en.wikipedia.org/wiki/Tower_of_Hanoi)
|
|
* [HackerRank Stacks and Queues Video](https://www.youtube.com/watch?v=wjI1WNcIntg)
|