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

• Difficulty Level : Easy
• Last Updated : 24 Jun, 2021

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

Examples:

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```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 string in below post.
Reverse a Stack using Recursion

The above solution requires O(n) extra space. We can reverse a string in O(1) time if we internally represent the stack as a linked list. Reverse a stack would require a reversing 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.

## 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 contribute 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```

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