Implement a stack using singly linked list

To implement a stack using a singly linked list, we need to ensure that all operations follow the LIFO (Last In, First Out) principle. This means that the most recently added element is always the first one to be removed. In this approach, we use a singly linked list, where each node contains data and a reference (or link) to the next node.

To manage the stack, we maintain a top pointer that always points to the most recent (topmost) node in the stack. The key stack operations—push, pop, and peek can be performed using this top pointer.

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

#include <bits/stdc++.h> using namespace std; class Node { public:  int data;  Node* next;  Node(int new_data) {  this->data = new_data;  this->next = nullptr;  } }; class Stack {  Node* head; public:  Stack() { this->head = nullptr; }  bool isEmpty() {  return head == nullptr;  }  void push(int new_data) {  Node* new_node = new Node(new_data);  if (!new_node) {  cout << "\nStack Overflow";  }  new_node->next = head;  head = new_node;  }  void pop() {  if (this->isEmpty()) {  cout << "\nStack Underflow" << endl;  } else {  Node* temp = head;  head = head->next;  delete temp;  }  }  int peek() {  if (!isEmpty())  return head->data;  else {  cout << "\nStack is empty";  return INT_MIN;  }  } }; int main() {  Stack st;  st.push(11);  st.push(22);  st.push(33);  st.push(44);  cout << "Top element is " << st.peek() << endl;  cout << "Removing two elements..." << endl;  st.pop();  st.pop();  cout << "Top element is " << st.peek() << endl;  return 0; } 

// C program to implement a stack using singly linked list #include <limits.h> #include <stdio.h> #include <stdlib.h> // Struct representing a node in the linked list typedef struct Node {  int data;  struct Node* next; } Node; Node* createNode(int new_data) {  Node* new_node = (Node*)malloc(sizeof(Node));  new_node->data = new_data;  new_node->next = NULL;  return new_node; } // Struct to implement stack using a singly linked list typedef struct Stack {  Node* head; } Stack; // Constructor to initialize the stack void initializeStack(Stack* stack) { stack->head = NULL; } // Function to check if the stack is empty int isEmpty(Stack* stack) {    // If head is NULL, the stack is empty  return stack->head == NULL; } // Function to push an element onto the stack void push(Stack* stack, int new_data) {    // Create a new node with given data  Node* new_node = createNode(new_data);  // Check if memory allocation for the new node failed  if (!new_node) {  printf("\nStack Overflow");  return;  }  // Link the new node to the current top node  new_node->next = stack->head;  // Update the top to the new node  stack->head = new_node; } // Function to remove the top element from the stack void pop(Stack* stack) {    // Check for stack underflow  if (isEmpty(stack)) {  printf("\nStack Underflow\n");  return;  }  else {    // Assign the current top to a temporary variable  Node* temp = stack->head;  // Update the top to the next node  stack->head = stack->head->next;  // Deallocate the memory of the old top node  free(temp);  } } // Function to return the top element of the stack int peek(Stack* stack) {    // If stack is not empty, return the top element  if (!isEmpty(stack))  return stack->head->data;  else {  printf("\nStack is empty");  return INT_MIN;  } } // Driver program to test the stack implementation int main() {    // Creating a stack  Stack stack;  initializeStack(&stack);  // Push elements onto the stack  push(&stack, 11);  push(&stack, 22);  push(&stack, 33);  push(&stack, 44);  // Print top element of the stack  printf("Top element is %d\n", peek(&stack));    // removing two elemements from the top  printf("Removing two elements...\n");  pop(&stack);  pop(&stack);  // Print top element of the stack  printf("Top element is %d\n", peek(&stack));  return 0; } 

// Java program to implement a stack using singly linked // list // Class representing a node in the linked list class Node {  int data;  Node next;  Node(int new_data) {  this.data = new_data;  this.next = null;  } } // Class to implement stack using a singly linked list class Stack {  // Head of the linked list  Node head;  // Constructor to initialize the stack  Stack() { this.head = null; }  // Function to check if the stack is empty  boolean isEmpty() {    // If head is null, the stack is empty  return head == null;  }  // Function to push an element onto the stack  void push(int new_data) {    // Create a new node with given data  Node new_node = new Node(new_data);  // Check if memory allocation for the new node  // failed  if (new_node == null) {  System.out.println("\nStack Overflow");  return;  }  // Link the new node to the current top node  new_node.next = head;  // Update the top to the new node  head = new_node;  }  // Function to remove the top element from the stack  void pop() {    // Check for stack underflow  if (isEmpty()) {  System.out.println("\nStack Underflow");  return;  }  else {    // Assign the current top to a temporary  // variable  Node temp = head;  // Update the top to the next node  head = head.next;  // Deallocate the memory of the old top node  temp = null;  }  }  // Function to return the top element of the stack  int peek() {    // If stack is not empty, return the top element  if (!isEmpty())  return head.data;  else {  System.out.println("\nStack is empty");  return Integer.MIN_VALUE;  }  } } // Driver code public class Main {  public static void main(String[] args)  {  // Creating a stack  Stack st = new Stack();  // Push elements onto the stack  st.push(11);  st.push(22);  st.push(33);  st.push(44);  // Print top element of the stack  System.out.println("Top element is " + st.peek());  // removing two elemements from the top  System.out.println("Removing two elements...");  st.pop();  st.pop();  // Print top element of the stack  System.out.println("Top element is " + st.peek());  } } 

# Java program to implement a stack using singly linked # list # Class representing a node in the linked list class Node: def __init__(self, new_data): self.data = new_data self.next = None # Class to implement stack using a singly linked list class Stack: def __init__(self): # head of the linked list self.head = None # Function to check if the stack is empty def is_empty(self): # If head is None, the stack is empty return self.head is None # Function to push an element onto the stack def push(self, new_data): # Create a new node with given data new_node = Node(new_data) # Check if memory allocation for the new node failed if not new_node: print("\nStack Overflow") return # Link the new node to the current top node new_node.next = self.head # Update the top to the new node self.head = new_node # Function to remove the top element from the stack def pop(self): # Check for stack underflow if self.is_empty(): print("\nStack Underflow") else: # Assign the current top to a temporary variable temp = self.head # Update the top to the next node self.head = self.head.next # Deallocate the memory of the old top node del temp # Function to return the top element of the stack def peek(self): # If stack is not empty, return the top element if not self.is_empty(): return self.head.data else: print("\nStack is empty") return float('-inf') # Creating a stack st = Stack() # Push elements onto the stack st.push(11) st.push(22) st.push(33) st.push(44) # Print top element of the stack print("Top element is", st.peek()) # removing two elemements from the top print("Removing two elements..."); st.pop() st.pop() # Print top element of the stack print("Top element is", st.peek()) 

// C# program to implement a stack using singly linked list using System; // Class representing a node in the linked list class Node {  public int data;  public Node next;  public Node(int new_data)  {  this.data = new_data;  this.next = null;  } } // Class to implement stack using a singly linked list class Stack {  // head of the linked list  private Node head;  // Constructor to initialize the stack  public Stack() { this.head = null; }  // Function to check if the stack is empty  public bool isEmpty()  {  // If head is null, the stack is empty  return head == null;  }  // Function to push an element onto the stack  public void push(int new_data)  {  // Create a new node with given data  Node new_node = new Node(new_data);  // Check if memory allocation for the new node  // failed  if (new_node == null) {  Console.WriteLine("\nStack Overflow");  return;  }  // Link the new node to the current top node  new_node.next = head;  // Update the top to the new node  head = new_node;  }  // Function to remove the top element from the stack  public void pop()  {  // Check for stack underflow  if (this.isEmpty()) {  Console.WriteLine("\nStack Underflow");  }  else {  // Update the top to the next node  head = head.next;  /* No need to manually free the memory of the  * old head in C# */  }  }  // Function to return the top element of the stack  public int peek()  {  // If stack is not empty, return the top element  if (!isEmpty())  return head.data;  else {  Console.WriteLine("\nStack is empty");  return int.MinValue;  }  } } // Driver program to test the stack implementation class GfG {  static void Main(string[] args)  {  // Creating a stack  Stack st = new Stack();  // Push elements onto the stack  st.push(11);  st.push(22);  st.push(33);  st.push(44);  // Print top element of the stack  Console.WriteLine("Top element is " + st.peek());  // removing two elemements from the top  Console.WriteLine("Removing two elements...");  st.pop();  st.pop();  // Print top element of the stack  Console.WriteLine("Top element is " + st.peek());  } } 

// Javascript program to implement a stack using singly // linked list // Class representing a node in the linked list class Node {  constructor(new_data) {  this.data = new_data;  this.next = null;  } } // Class to implement stack using a singly linked list class Stack {  // Constructor to initialize the stack  constructor() { this.head = null; }  // Function to check if the stack is empty  isEmpty() {    // If head is null, the stack is empty  return this.head === null;  }  // Function to push an element onto the stack  push(new_data) {    // Create a new node with given data  const new_node = new Node(new_data);  // Check if memory allocation for the new node  // failed  if (!new_node) {  console.log("\nStack Overflow");  return;  }  // Link the new node to the current top node  new_node.next = this.head;  // Update the top to the new node  this.head = new_node;  }  // Function to remove the top element from the stack  pop() {    // Check for stack underflow  if (this.isEmpty()) {  console.log("\nStack Underflow");  }  else {    // Assign the current top to a temporary  // variable  let temp = this.head;  // Update the top to the next node  this.head = this.head.next;  // Deallocate the memory of the old top node  temp = null;  }  }  // Function to return the top element of the stack  peek() {    // If stack is not empty, return the top element  if (!this.isEmpty())  return this.head.data;  else {  console.log("\nStack is empty");  return Number.MIN_VALUE;  }  } } // Driver program to test the stack implementation const st = new Stack(); // Push elements onto the stack st.push(11); st.push(22); st.push(33); st.push(44); // Print top element of the stack console.log("Top element is " + st.peek()); // removing two elemements from the top console.log("Removing two elements..."); st.pop(); st.pop(); // Print top element of the stack console.log("Top element is " + st.peek()); 

Top element is 44 Removing two elements... 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.
Auxiliary Space: O(n), where n is the size of the stack

Benefits of implementing a stack using a singly linked list

• Dynamic memory allocation: The size of the stack can be increased or decreased dynamically by adding or removing nodes from the linked list, without the need to allocate a fixed amount of memory for the stack upfront.
• Efficient memory usage: Since nodes in a singly linked list only have a next pointer and not a prev pointer, they use less memory than nodes in a doubly linked list.
• Easy implementation: Implementing a stack using a singly linked list is straightforward and can be done using just a few lines of code.
• Versatile: Singly linked lists can be used to implement other data structures such as queues, linked lists, and trees.

Real time examples of stack

Stacks are used in various real-world scenarios where a last-in, first-out (LIFO) data structure is required. Here are some examples of real-time applications of stacks:

• Function Call Stack: When a function is called, its return address and parameters are pushed onto the stack. The stack ensures functions execute and return in reverse order..
• Undo/Redo Operations: In apps like text or image editors, actions are pushed onto a stack. Undo removes the last action, while redo restores it.
• Browser History: Browsers use stacks to track visited pages. Visiting a page pushes its URL onto the stack, and the "Back" button pops the last URL to go to the previous page.
• Expression Evaluation: In compilers, expressions are converted to postfix notation and evaluated using a stack.
• Call Stack in Recursion: Recursive function calls are pushed onto the stack. Once recursion ends, the stack is popped to return to the previous function call.