Design a stack with the following operations.
- push(Stack s, x): Adds an item x to stack s
- pop(Stack s): Removes the top item from stack s
- merge(Stack s1, Stack s2): Merge contents of s2 into s1.
Time Complexity of all above operations should be O(1).
If we use array implementation of the stack, then merge is not possible to do in O(1) time as we have to do the following steps.
- Delete old arrays.
- Create a new array for s1 with a size equal to the size of the old array for s1 plus size of s2.
- Copy old contents of s1 and s2 to new array for s1
The above operations take O(n) time.
We can use a linked list with two pointers, one pointer to the first node (also used as a top when elements are added and removed from the beginning). The other pointer is needed for the last node so that we can quickly link the linked list of s2 at the end of s1. Following are all operations.
- push(): Adds the new item at the beginning of linked list using the first pointer.
- pop(): Removes an item from the beginning using the first pointer.
- merge(): Links the first pointer second stack as next of the last pointer of the first list.
Can we do it if we are not allowed to use an extra pointer?
We can do it with a circular linked list. The idea is to keep track of the last node in the linked list. The next of the last node indicates the top of the stack.
- push(): Adds the new item as next of the last node.
- pop(): Removes next of last node.
- merge(): Links the top (next of last) of the second list to the top (next of last) of the first list. And makes last of the second list as last of the whole list.
The code for the above is given below:
C++
#include <iostream>
using namespace std;
class node {
public:
int data;
node* next;
};
class mystack {
public:
node* head;
node* tail;
mystack()
{
head = NULL;
tail = NULL;
}
};
mystack* create()
{
mystack* ms = new mystack();
return ms;
}
void push(int data, mystack* ms)
{
node* temp = new node();
temp->data = data;
temp->next = ms->head;
if (ms->head == NULL)
ms->tail = temp;
ms->head = temp;
}
int pop(mystack* ms)
{
if (ms->head == NULL) {
cout << "stack underflow" << endl;
return 0;
}
else {
node* temp = ms->head;
ms->head = ms->head->next;
int popped = temp->data;
delete temp;
return popped;
}
}
void merge(mystack* ms1, mystack* ms2)
{
if (ms1->head == NULL)
{
ms1->head = ms2->head;
ms1->tail = ms2->tail;
return;
}
ms1->tail->next = ms2->head;
ms1->tail = ms2->tail;
}
void display(mystack* ms)
{
node* temp = ms->head;
while (temp != NULL) {
cout << temp->data << " ";
temp = temp->next;
}
}
int main()
{
mystack* ms1 = create();
mystack* ms2 = create();
push(6, ms1);
push(5, ms1);
push(4, ms1);
push(9, ms2);
push(8, ms2);
push(7, ms2);
merge(ms1, ms2);
display(ms1);
}
|
Java
import java.io.*;
class Node
{
Node next;
Node prev;
int data;
Node(int value)
{
data = value;
next = null;
prev = null;
}
}
class Stack{
private Node head;
private Node tail;
Stack()
{
head = null;
tail = null;
}
public void push(int value)
{
Node newNode = new Node(value);
if(head == null)
{
head = newNode;
head.next=null;
head.prev = null;
tail = newNode;
}
else
{
newNode.prev = tail;
tail.next = newNode;
tail = newNode;
}
}
public void pop()
{
if(head == null)
System.out.println("stack underflow");
if(head == tail)
{
head = null;
tail = null;
}
else
{
Node n = tail;
tail = tail.prev;
n.prev = null;
tail.next = null;
}
}
public void merge(Stack s)
{
head.prev = s.tail;
s.tail.next = head;
head = s.head;
s.tail = null;
s.head = null;
}
public void display()
{
if(tail != null)
{
Node n = tail;
while(n != null)
{
System.out.print(n.data + " ");
n = n.prev;
}
System.out.println();
}
else
{
System.out.println("Stack Underflow");
}
}
}
class GFG{
public static void main (String[] args)
{
Stack ms1 = new Stack();
Stack ms2 = new Stack();
ms1.push(6);
ms1.push(5);
ms1.push(4);
ms2.push(9);
ms2.push(8);
ms2.push(7);
ms1.merge(ms2);
ms1.display();
}
}
|
Python3
class Node():
def __init__(self,data):
self.next = None
self.prev = None
self.data = data
class Stack():
def __init__(self):
self.head = None
self.tail = None
def push(self, data):
new_node = Node(data)
if (self.head == None):
self.head = new_node
self.head.next= None
self.head.prev = None
self.tail = new_node
else:
new_node.prev = self.tail
self.tail.next = new_node
self.tail = new_node
def pop(self):
if (self.head == None):
print("Stack underflow")
if (self.head == self.tail):
self.head = None
self.tail = None
else:
node = self.tail
self.tail = self.tail.prev
del node
self.tail.next = None
def merge(self, stack):
if stack.head == None: return
if self.head == None:
self.head = stack.head
self.tail = stack.tail
return
self.head.prev = stack.tail
stack.tail.nxt = self.head
self.head = stack.head
def display(self):
if (self.tail != None):
n = self.tail
while (n != None):
print(n.data, end = " ")
n = n.prev
print()
else:
print("Stack Underflow")
ms1 = Stack()
ms2 = Stack()
ms1.push(6)
ms1.push(5)
ms1.push(4)
ms2.push(9)
ms2.push(8)
ms2.push(7)
ms1.merge(ms2)
ms1.display()
while ms1.head != ms1.tail:
ms1.pop ()
print ("check pop all elements until head == tail (one element left)")
print ("on merged stack: ", end = "")
ms1.display()
|
C#
using System;
public
class Node {
public
Node next;
public
Node prev;
public
int data;
public
Node(int value)
{
data = value;
next = null;
prev = null;
}
}
public
class Stack {
private Node head;
private Node tail;
public Stack()
{
head = null;
tail = null;
}
public void Push(int value)
{
Node newNode = new Node(value);
if (head == null) {
head = newNode;
head.next = null;
head.prev = null;
tail = newNode;
}
else {
newNode.prev = tail;
tail.next = newNode;
tail = newNode;
}
}
public void Pop()
{
if (head == null)
Console.WriteLine("stack underflow");
if (head == tail) {
head = null;
tail = null;
}
else {
Node n = tail;
tail = tail.prev;
n.prev = null;
tail.next = null;
}
}
public void merge(Stack s)
{
head.prev = s.tail;
s.tail.next = head;
head = s.head;
s.tail = null;
s.head = null;
}
public void display()
{
if (tail != null) {
Node n = tail;
while (n != null) {
Console.Write(n.data + " ");
n = n.prev;
}
Console.WriteLine();
}
else {
Console.WriteLine("Stack Underflow");
}
}
}
public class GFG {
public static void Main(String[] args)
{
Stack ms1 = new Stack();
Stack ms2 = new Stack();
ms1.Push(6);
ms1.Push(5);
ms1.Push(4);
ms2.Push(9);
ms2.Push(8);
ms2.Push(7);
ms1.merge(ms2);
ms1.display();
}
}
|
Javascript
class node {
constructor() {
this.data;
this.next;
}
}
class mystack {
constructor() {
this.head = null;
this.tail = null;
}
}
function create() {
var ms = new mystack();
return ms;
}
function push(data, ms) {
var temp = new node();
temp.data = data;
temp.next = ms.head;
if (ms.head == null) ms.tail = temp;
ms.head = temp;
}
function pop(ms) {
if (ms.head == null) {
console.log("stack underflow");
return 0;
} else {
temp = ms.head;
ms.head = ms.head.next;
var popped = temp.data;
delete temp;
return popped;
}
}
function merge(ms1, ms2) {
if (ms1.head == null) {
ms1.head = ms2.head;
ms1.tail = ms2.tail;
return;
}
ms1.tail.next = ms2.head;
ms1.tail = ms2.tail;
}
function display(ms) {
temp = ms.head;
while (temp != null) {
console.log(temp.data + " ");
temp = temp.next;
}
}
ms1 = create();
ms2 = create();
push(6, ms1);
push(5, ms1);
push(4, ms1);
push(9, ms2);
push(8, ms2);
push(7, ms2);
merge(ms1, ms2);
display(ms1);
|
Time Complexity : O(1) , as all operation use constant time.
Auxiliary Space : O(1) , since no extra space used.