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

• Difficulty Level : Easy
• Last Updated : 30 Mar, 2022

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 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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