Implement a stack using singly linked list

To implement a stack using the singly linked list concept, all the singly linked list operations should be performed based on Stack operations LIFO(last in first out) and with the help of that knowledge, we are going to implement a stack using a singly linked list. 

So we need to follow a simple rule in the implementation of a stack which is last in first out and all the operations can be performed with the help of a top variable. Let us learn how to perform Pop, Push, Peek, and Display operations in the following article:

In the stack Implementation, a stack contains a top pointer. which is the “head” of the stack where pushing and popping items happens at the head of the list. The first node has a null in the link field and second node-link has the first node address in the link field and so on and the last node address is in the “top” pointer.

The main advantage of using a linked list over arrays is that it is possible to implement a stack that can shrink or grow as much as needed. Using an array will put a restriction on the maximum capacity of the array which can lead to stack overflow. Here each new node will be dynamically allocated. so overflow is not possible.

Stack Operations:

• push(): Insert a new element into the stack i.e just insert a new element at the beginning of the linked list.
• pop(): Return the top element of the Stack i.e simply delete the first element from the linked list.
• peek(): Return the top element.
• display(): Print all elements in Stack.

Push Operation:

• Initialise a node
• Update the value of that node by data i.e. node->data = data
• Now link this node to the top of the linked list
• And update top pointer to the current node

Pop Operation:

• First Check whether there is any node present in the linked list or not, if not then return
• Otherwise make pointer let say temp to the top node and move forward the top node by 1 step
• Now free this temp node

Peek Operation:

• Check if there is any node present or not, if not then return.
• Otherwise return the value of top node of the linked list

Display Operation:

• Take a temp node and initialize it with top pointer 
• Now start traversing temp till it encounters NULL
• Simultaneously print the value of the temp node

Below is the implementation of the above operations 

C++

// C++ program to Implement a stack
// using singly linked list
#include <bits/stdc++.h>
using namespace std;

// creating a linked list;
class Node {
public:
    int data;
    Node* link;

    // Constructor
    Node(int n)
    {
        this->data = n;
        this->link = NULL;
    }
};

class Stack {
    Node* top;

public:
    Stack() { top = NULL; }

    void push(int data)
    {

        // Create new node temp and allocate memory in heap
        Node* temp = new Node(data);

        // Check if stack (heap) is full.
        // Then inserting an element would
        // lead to stack overflow
        if (!temp) {
            cout << "\nStack Overflow";
            exit(1);
        }

        // Initialize data into temp data field
        temp->data = data;

        // Put top pointer reference into temp link
        temp->link = top;

        // Make temp as top of Stack
        top = temp;
    }

    // Utility function to check if
    // the stack is empty or not
    bool isEmpty()
    {
        // If top is NULL it means that
        // there are no elements are in stack
        return top == NULL;
    }

    // Utility function to return top element in a stack
    int peek()
    {
        // If stack is not empty , return the top element
        if (!isEmpty())
            return top->data;
        else
            exit(1);
    }

    // Function to remove
    // a key from given queue q
    void pop()
    {
        Node* temp;

        // Check for stack underflow
        if (top == NULL) {
            cout << "\nStack Underflow" << endl;
            exit(1);
        }
        else {

            // Assign top to temp
            temp = top;

            // Assign second node to top
            top = top->link;

            // This will automatically destroy
            // the link between first node and second node

            // Release memory of top node
            // i.e delete the node
            free(temp);
        }
    }

    // Function to print all the
    // elements of the stack
    void display()
    {
        Node* temp;

        // Check for stack underflow
        if (top == NULL) {
            cout << "\nStack Underflow";
            exit(1);
        }
        else {
            temp = top;
            while (temp != NULL) {

                // Print node data
                cout << temp->data;

                // Assign temp link to temp
                temp = temp->link;
                if (temp != NULL)
                    cout << " -> ";
            }
        }
    }
};

// Driven Program
int main()
{
    // Creating a stack
    Stack s;

    // Push the elements of stack
    s.push(11);
    s.push(22);
    s.push(33);
    s.push(44);

    // Display stack elements
    s.display();

    // Print top element of stack
    cout << "\nTop element is " << s.peek() << endl;

    // Delete top elements of stack
    s.pop();
    s.pop();

    // Display stack elements
    s.display();

    // Print top element of stack
    cout << "\nTop element is " << s.peek() << endl;

    return 0;
}

Java

// Java program to Implement a stack
// using singly linked list
// import package
import static java.lang.System.exit;

// Driver code
class GFG {
    public static void main(String[] args)
    {
        // create Object of Implementing class
        StackUsingLinkedlist obj
            = new StackUsingLinkedlist();
        // insert Stack value
        obj.push(11);
        obj.push(22);
        obj.push(33);
        obj.push(44);

        // print Stack elements
        obj.display();

        // print Top element of Stack
        System.out.printf("\nTop element is %d\n",
                          obj.peek());

        // Delete top element of Stack
        obj.pop();
        obj.pop();

        // print Stack elements
        obj.display();

        // print Top element of Stack
        System.out.printf("\nTop element is %d\n",
                          obj.peek());
    }
}

// Create Stack Using Linked list
class StackUsingLinkedlist {

    // A linked list node
    private class Node {

        int data; // integer data
        Node link; // reference variable Node type
    }
    // create global top reference variable global
    Node top;
    // Constructor
    StackUsingLinkedlist() { this.top = null; }

    // Utility function to add an element x in the stack
    public void push(int x) // insert at the beginning
    {
        // create new node temp and allocate memory
        Node temp = new Node();

        // check if stack (heap) is full. Then inserting an
        //  element would lead to stack overflow
        if (temp == null) {
            System.out.print("\nHeap Overflow");
            return;
        }

        // initialize data into temp data field
        temp.data = x;

        // put top reference into temp link
        temp.link = top;

        // update top reference
        top = temp;
    }

    // Utility function to check if the stack is empty or
    // not
    public boolean isEmpty() { return top == null; }

    // Utility function to return top element in a stack
    public int peek()
    {
        // check for empty stack
        if (!isEmpty()) {
            return top.data;
        }
        else {
            System.out.println("Stack is empty");
            return -1;
        }
    }

    // Utility function to pop top element from the stack
    public void pop() // remove at the beginning
    {
        // check for stack underflow
        if (top == null) {
            System.out.print("\nStack Underflow");
            return;
        }

        // update the top pointer to point to the next node
        top = (top).link;
    }

    public void display()
    {
        // check for stack underflow
        if (top == null) {
            System.out.printf("\nStack Underflow");
            exit(1);
        }
        else {
            Node temp = top;
            while (temp != null) {

                // print node data
                System.out.print(temp.data);

                // assign temp link to temp
                temp = temp.link;
                if(temp != null)
                    System.out.print(" -> ");
            }
        }
    }
}

Python3

# python3 program to Implement a stack
# using singly linked list

class Node:

    # Class to create nodes of linked list
    # constructor initializes node automatically
    def __init__(self, data):
        self.data = data
        self.next = None


class Stack:

    # head is default NULL
    def __init__(self):
        self.head = None

    # Checks if stack is empty
    def isempty(self):
        if self.head == None:
            return True
        else:
            return False

    # Method to add data to the stack
    # adds to the start of the stack
    def push(self, data):

        if self.head == None:
            self.head = Node(data)

        else:
            newnode = Node(data)
            newnode.next = self.head
            self.head = newnode

    # Remove element that is the current head (start of the stack)
    def pop(self):

        if self.isempty():
            return None

        else:
            # Removes the head node and makes
            # the preceding one the new head
            poppednode = self.head
            self.head = self.head.next
            poppednode.next = None
            return poppednode.data

    # Returns the head node data
    def peek(self):

        if self.isempty():
            return None

        else:
            return self.head.data

    # Prints out the stack
    def display(self):

        iternode = self.head
        if self.isempty():
            print("Stack Underflow")

        else:

            while(iternode != None):

                print(iternode.data, end = "")
                iternode = iternode.next
                if(iternode != None):
                    print(" -> ", end = "")
            return


# Driver code
if __name__ == "__main__":
  MyStack = Stack()

  MyStack.push(11)
  MyStack.push(22)
  MyStack.push(33)
  MyStack.push(44)

  # Display stack elements
  MyStack.display()

  # Print top element of stack
  print("\nTop element is ", MyStack.peek())

  # Delete top elements of stack
  MyStack.pop()
  MyStack.pop()

  # Display stack elements
  MyStack.display()

  # Print top element of stack
  print("\nTop element is ", MyStack.peek())

# This code is contributed by Mathew George

C#

// C# program to Implement a stack
// using singly linked list

// import package
using System;

// Create Stack Using Linked list
public class StackUsingLinkedlist {

    // A linked list node
    private class Node {
        // integer data
        public int data;

        // reference variable Node type
        public Node link;
    }

    // create global top reference variable
    Node top;

    // Constructor
    public StackUsingLinkedlist() { this.top = null; }

    // Utility function to add
    // an element x in the stack
    // insert at the beginning
    public void push(int x)
    {
        // create new node temp and allocate memory
        Node temp = new Node();

        // check if stack (heap) is full.
        // Then inserting an element
        // would lead to stack overflow
        if (temp == null) {
            Console.Write("\nHeap Overflow");
            return;
        }

        // initialize data into temp data field
        temp.data = x;

        // put top reference into temp link
        temp.link = top;

        // update top reference
        top = temp;
    }

    // Utility function to check if
    // the stack is empty or not
    public bool isEmpty() { return top == null; }

    // Utility function to return
    // top element in a stack
    public int peek()
    {
        // check for empty stack
        if (!isEmpty()) {
            return top.data;
        }
        else {
            Console.WriteLine("Stack is empty");
            return -1;
        }
    }

    // Utility function to pop top element from the stack
    public void pop() // remove at the beginning
    {
        // check for stack underflow
        if (top == null) {
            Console.Write("\nStack Underflow");
            return;
        }

        // update the top pointer to
        // point to the next node
        top = (top).link;
    }

    public void display()
    {
        // check for stack underflow
        if (top == null) {
            Console.Write("\nStack Underflow");
            return;
        }
        else {
            Node temp = top;
            while (temp != null) {

                // print node data
                Console.Write(temp.data);

                // assign temp link to temp
                temp = temp.link;
                if(temp != null)
                    Console.Write(" -> ");
            }
        }
    }
}

// Driver code
public class GFG {
    public static void Main(String[] args)
    {
        // create Object of Implementing class
        StackUsingLinkedlist obj
            = new StackUsingLinkedlist();

        // insert Stack value
        obj.push(11);
        obj.push(22);
        obj.push(33);
        obj.push(44);

        // print Stack elements
        obj.display();

        // print Top element of Stack
        Console.Write("\nTop element is {0}\n", obj.peek());

        // Delete top element of Stack
        obj.pop();
        obj.pop();

        // print Stack elements
        obj.display();

        // print Top element of Stack
        Console.Write("\nTop element is {0}\n", obj.peek());
    }
}

// This code is contributed by 29AjayKumar

Javascript

// Javascript program to Implement a stack
// using singly linked list
// import package

// A linked list node
class Node
{
    constructor()
    {
        this.data=0;
        this.link=null;
    }
}

// Create Stack Using Linked list
class StackUsingLinkedlist
{
    constructor()
    {
        this.top=null;
    }

    // Utility function to add an element x in the stack
    push(x)
    {
        // create new node temp and allocate memory
        let temp = new Node();

        // check if stack (heap) is full. Then inserting an
        //  element would lead to stack overflow
        if (temp == null) {
            document.write("<br>Heap Overflow");
            return;
        }

        // initialize data into temp data field
        temp.data = x;

        // put top reference into temp link
        temp.link = this.top;

        // update top reference
        this.top = temp;
    }

     // Utility function to check if the stack is empty or not
    isEmpty()
    {
         return this.top == null;
    }

    // Utility function to return top element in a stack
    peek()
    {
        // check for empty stack
        if (!this.isEmpty()) {
            return this.top.data;
        }
        else {
            document.write("Stack is empty<br>");
            return -1;
        }
    }

    // Utility function to pop top element from the stack
    pop() // remove at the beginning
    {
        // check for stack underflow
        if (this.top == null) {
            document.write("<br>Stack Underflow");
            return;
        }

        // update the top pointer to point to the next node
        this.top = this.top.link;
    }

    display()
    {
        // check for stack underflow
        if (this.top == null) {
            document.write("<br>Stack Underflow");

        }
        else {
            let temp = this.top;
            while (temp != null) {

                // print node data
                document.write(temp.data+"->");

                // assign temp link to temp
                temp = temp.link;
            }
        }
    }
}

// main class

// create Object of Implementing class
let obj = new StackUsingLinkedlist();
// insert Stack value
obj.push(11);
obj.push(22);
obj.push(33);
obj.push(44);

// print Stack elements
obj.display();

// print Top element of Stack
document.write("<br>Top element is ", obj.peek()+"<br>");

// Delete top element of Stack
obj.pop();
obj.pop();

// print Stack elements
obj.display();

// print Top element of Stack
document.write("<br>Top element is ", obj.peek()+"<br>");

// This code is contributed by rag2127

Output

44 -> 33 -> 22 -> 11
Top element is 44
22 -> 11
Top element is 22

Time Complexity: O(1), for all push(), pop(), and peek(), as we are not performing any kind of traversal over the list. We perform all the operations through the current pointer only.
Auxiliary Space: O(N), where N is the size of the stack