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# Reverse a stack without using extra space in O(n)

• Difficulty Level : Easy
• Last Updated : 10 Jan, 2023

Reverse a Stack without using recursion and extra space. Even the functional Stack is not allowed.

Examples:

```Input : 1->2->3->4
Output : 4->3->2->1

Input :  6->5->4
Output : 4->5->6```

We have discussed a way of reversing a stack in the below post.
Reverse a Stack using Recursion

The above solution requires O(n) extra space. We can reverse a stack in O(1) time if we internally represent the stack as a linked list. Reverse a stack would require a reversing of a linked list which can be done with O(n) time and O(1) extra space.
Note that push() and pop() operations still take O(1) time.

Implementation:

## C++

 `// C++ program to implement Stack  ` `// using linked list so that reverse ` `// can be done with O(1) extra space. ` `#include ` `using` `namespace` `std; ` ` `  `class` `StackNode { ` `    ``public``: ` `    ``int` `data; ` `    ``StackNode *next; ` `     `  `    ``StackNode(``int` `data) ` `    ``{ ` `        ``this``->data = data; ` `        ``this``->next = NULL; ` `    ``} ` `}; ` ` `  `class` `Stack { ` `     `  `    ``StackNode *top; ` `     `  `    ``public``: ` `     `  `    ``// Push and pop operations ` `    ``void` `push(``int` `data) ` `    ``{ ` `        ``if` `(top == NULL) { ` `            ``top = ``new` `StackNode(data); ` `            ``return``; ` `        ``} ` `        ``StackNode *s = ``new` `StackNode(data); ` `        ``s->next = top; ` `        ``top = s; ` `    ``} ` `     `  `    ``StackNode* pop() ` `    ``{ ` `        ``StackNode *s = top; ` `        ``top = top->next; ` `        ``return` `s; ` `    ``} ` ` `  `    ``// prints contents of stack ` `    ``void` `display() ` `    ``{ ` `        ``StackNode *s = top; ` `        ``while` `(s != NULL) { ` `            ``cout << s->data << ``" "``; ` `            ``s = s->next; ` `        ``} ` `        ``cout << endl; ` `    ``} ` ` `  `    ``// Reverses the stack using simple ` `    ``// linked list reversal logic. ` `    ``void` `reverse() ` `    ``{ ` `        ``StackNode *prev, *cur, *succ; ` `        ``cur = prev = top; ` `        ``cur = cur->next; ` `        ``prev->next = NULL; ` `        ``while` `(cur != NULL) { ` ` `  `            ``succ = cur->next; ` `            ``cur->next = prev; ` `            ``prev = cur; ` `            ``cur = succ; ` `        ``} ` `        ``top = prev; ` `    ``} ` `}; ` ` `  `// driver code ` `int` `main() ` `{ ` `    ``Stack *s = ``new` `Stack(); ` `    ``s->push(1); ` `    ``s->push(2); ` `    ``s->push(3); ` `    ``s->push(4); ` `    ``cout << ``"Original Stack"` `<< endl;; ` `    ``s->display(); ` `    ``cout << endl; ` `     `  `    ``// reverse ` `    ``s->reverse(); ` ` `  `    ``cout << ``"Reversed Stack"` `<< endl; ` `    ``s->display(); ` `     `  `    ``return` `0; ` `} ` `// This code is contributed by Chhavi. `

## Java

 `// Java program to implement Stack using linked ` `// list so that reverse can be done with O(1)  ` `// extra space. ` `class` `StackNode { ` `    ``int` `data; ` `    ``StackNode next; ` `    ``public` `StackNode(``int` `data) ` `    ``{ ` `        ``this``.data = data; ` `        ``this``.next = ``null``; ` `    ``} ` `} ` ` `  `class` `Stack { ` `    ``StackNode top; ` ` `  `    ``// Push and pop operations ` `    ``public` `void` `push(``int` `data) ` `    ``{ ` `        ``if` `(``this``.top == ``null``) { ` `            ``top = ``new` `StackNode(data); ` `            ``return``; ` `        ``} ` `        ``StackNode s = ``new` `StackNode(data); ` `        ``s.next = ``this``.top; ` `        ``this``.top = s; ` `    ``} ` `    ``public` `StackNode pop() ` `    ``{ ` `        ``StackNode s = ``this``.top; ` `        ``this``.top = ``this``.top.next; ` `        ``return` `s; ` `    ``} ` ` `  `    ``// prints contents of stack ` `    ``public` `void` `display() ` `    ``{ ` `        ``StackNode s = ``this``.top; ` `        ``while` `(s != ``null``) { ` `            ``System.out.print(s.data + ``" "``); ` `            ``s = s.next; ` `        ``} ` `        ``System.out.println(); ` `    ``} ` ` `  `    ``// Reverses the stack using simple ` `    ``// linked list reversal logic. ` `    ``public` `void` `reverse() ` `    ``{ ` `        ``StackNode prev, cur, succ; ` `        ``cur = prev = ``this``.top; ` `        ``cur = cur.next; ` `        ``prev.next = ``null``; ` `        ``while` `(cur != ``null``) { ` ` `  `            ``succ = cur.next; ` `            ``cur.next = prev; ` `            ``prev = cur; ` `            ``cur = succ; ` `        ``} ` `        ``this``.top = prev; ` `    ``} ` `} ` ` `  `public` `class` `reverseStackWithoutSpace { ` `    ``public` `static` `void` `main(String[] args) ` `    ``{ ` `        ``Stack s = ``new` `Stack(); ` `        ``s.push(``1``); ` `        ``s.push(``2``); ` `        ``s.push(``3``); ` `        ``s.push(``4``); ` `        ``System.out.println(``"Original Stack"``); ` `        ``s.display(); ` ` `  `        ``// reverse ` `        ``s.reverse(); ` ` `  `        ``System.out.println(``"Reversed Stack"``); ` `        ``s.display(); ` `    ``} ` `} `

## Python3

 `# Python3 program to implement Stack  ` `# using linked list so that reverse ` `# can be done with O(1) extra space. ` `class` `StackNode: ` `     `  `    ``def` `__init__(``self``, data): ` `         `  `        ``self``.data ``=` `data ` `        ``self``.``next` `=` `None` ` `  `class` `Stack: ` `     `  `    ``def` `__init__(``self``): ` `          `  `        ``self``.top ``=` `None` `      `  `    ``# Push and pop operations ` `    ``def` `push(``self``, data): ` `     `  `        ``if` `(``self``.top ``=``=` `None``): ` `            ``self``.top ``=` `StackNode(data) ` `            ``return` `         `  `        ``s ``=` `StackNode(data) ` `        ``s.``next` `=` `self``.top ` `        ``self``.top ``=` `s ` `      `  `    ``def` `pop(``self``): ` `     `  `        ``s ``=` `self``.top ` `        ``self``.top ``=` `self``.top.``next` `        ``return` `s ` `  `  `    ``# Prints contents of stack ` `    ``def` `display(``self``): ` `     `  `        ``s ``=` `self``.top ` `         `  `        ``while` `(s !``=` `None``): ` `            ``print``(s.data, end ``=` `' '``) ` `            ``s ``=` `s.``next` `         `  `    ``# Reverses the stack using simple ` `    ``# linked list reversal logic. ` `    ``def` `reverse(``self``): ` ` `  `        ``prev ``=` `self``.top ` `        ``cur ``=` `self``.top ` `        ``cur ``=` `cur.``next` `        ``succ ``=` `None` `        ``prev.``next` `=` `None` `         `  `        ``while` `(cur !``=` `None``): ` `            ``succ ``=` `cur.``next` `            ``cur.``next` `=` `prev ` `            ``prev ``=` `cur ` `            ``cur ``=` `succ ` `         `  `        ``self``.top ``=` `prev ` `     `  `# Driver code ` `if` `__name__``=``=``'__main__'``: ` `     `  `    ``s ``=` `Stack() ` `    ``s.push(``1``) ` `    ``s.push(``2``) ` `    ``s.push(``3``) ` `    ``s.push(``4``) ` `     `  `    ``print``(``"Original Stack"``) ` `    ``s.display() ` `    ``print``() ` `      `  `    ``# Reverse ` `    ``s.reverse() ` `  `  `    ``print``(``"Reversed Stack"``) ` `    ``s.display() ` `      `  `# This code is contributed by rutvik_56`

## C#

 `// C# program to implement Stack using linked ` `// list so that reverse can be done with O(1)  ` `// extra space. ` `using` `System;  ` ` `  `public` `class` `StackNode ` `{ ` `    ``public` `int` `data; ` `    ``public` `StackNode next; ` `    ``public` `StackNode(``int` `data) ` `    ``{ ` `        ``this``.data = data; ` `        ``this``.next = ``null``; ` `    ``} ` `} ` ` `  `public` `class` `Stack  ` `{ ` `    ``public` `StackNode top; ` ` `  `    ``// Push and pop operations ` `    ``public` `void` `push(``int` `data) ` `    ``{ ` `        ``if` `(``this``.top == ``null``) ` `        ``{ ` `            ``top = ``new` `StackNode(data); ` `            ``return``; ` `        ``} ` `        ``StackNode s = ``new` `StackNode(data); ` `        ``s.next = ``this``.top; ` `        ``this``.top = s; ` `    ``} ` `     `  `    ``public` `StackNode pop() ` `    ``{ ` `        ``StackNode s = ``this``.top; ` `        ``this``.top = ``this``.top.next; ` `        ``return` `s; ` `    ``} ` ` `  `    ``// prints contents of stack ` `    ``public` `void` `display() ` `    ``{ ` `        ``StackNode s = ``this``.top; ` `        ``while` `(s != ``null``)  ` `        ``{ ` `            ``Console.Write(s.data + ``" "``); ` `            ``s = s.next; ` `        ``} ` `        ``Console.WriteLine(); ` `    ``} ` ` `  `    ``// Reverses the stack using simple ` `    ``// linked list reversal logic. ` `    ``public` `void` `reverse() ` `    ``{ ` `        ``StackNode prev, cur, succ; ` `        ``cur = prev = ``this``.top; ` `        ``cur = cur.next; ` `        ``prev.next = ``null``; ` `        ``while` `(cur != ``null``) ` `        ``{ ` `            ``succ = cur.next; ` `            ``cur.next = prev; ` `            ``prev = cur; ` `            ``cur = succ; ` `        ``} ` `        ``this``.top = prev; ` `    ``} ` `} ` ` `  `public` `class` `reverseStackWithoutSpace  ` `{ ` `    ``// Driver code ` `    ``public` `static` `void` `Main(String []args) ` `    ``{ ` `        ``Stack s = ``new` `Stack(); ` `        ``s.push(1); ` `        ``s.push(2); ` `        ``s.push(3); ` `        ``s.push(4); ` `        ``Console.WriteLine(``"Original Stack"``); ` `        ``s.display(); ` ` `  `        ``// reverse ` `        ``s.reverse(); ` ` `  `        ``Console.WriteLine(``"Reversed Stack"``); ` `        ``s.display(); ` `    ``} ` `} ` ` `  `// This code is contributed by Arnab Kundu `

## Javascript

 ``

Output

```Original Stack
4 3 2 1

Reversed Stack
1 2 3 4 ```

Time Complexity: O(n), as we are using a loop to traverse n times. Where n is the number of nodes in the linked list.
Auxiliary Space: O(1), as we are not using any extra space.

This article is contributed by Niharika Sahai. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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