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

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
```

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

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 inked 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++

```// CPP program to implement Stack
// using linked list so that reverse
// can be done with O(1) extra space.
#include<bits/stdc++.h>
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();
}
}
```

Output:

```Original Stack
4 3 2 1
Reversed Stack
1 2 3 4
```

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

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