freeCodeCamp/guide/english/computer-science/data-structures/stacks/index.md

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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
5. Top() - Return the item at the top of the stack without popping. (All the operations can be done in O(1) time - depending on the implementation) Implementation of a stack is possible using arrays, linked lists or other dynamic collections such as array lists (Java) or vectors (C++). When dealing with dynamic collections, it is important to insert items at the end to prevent shifting and maintain an O(1) runtime on each operation.

The following is a simple array implementation of the stack data structure with its most common operations.

//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;
}

Stack implementation using STL in C++

#include <iostream>
#include <stack>

using namespace std;

int main() {
    stack<int> s;
    s.push(10);
    s.push(20);
    s.push(30);
    cout << "The size of the stack is: " << s.size() << endl;
    cout << "Top of the stack is: " << s.top() << endl;
    cout << "The stack is: " << endl;
    while (!s.empty()) {
        cout << s.top() << " ";
        s.pop();
    }
    cout << endl;
}

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.

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.

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
• [Stack - Wikipedia](https://en.wikipedia.org/wiki/Stack_(abstract_data_type)
• Tower of Hanoi Problem and how the solution uses stacks and recursions
• HackerRank Stacks and Queues Video