# C Algorithm – Reverse a stack using recursion

C Algorithm - Reverse a stack using recursion - Data Structure - Write a program to reverse a stack using recursion. You are not allowed to use

Write a program to reverse a stack using recursion. You are not allowed to use loop constructs like while, for..etc, and you can only use the following ADT functions on Stack S:
isEmpty(S)
push(S)
pop(S)

Solution:
The idea of the solution is to hold all values in Function Call Stack until the stack becomes empty. When the stack becomes empty, insert all held items one by one at the bottom of the stack.

For example, let the input stack be

```    1  <-- top
2
3
4
```
```First 4 is inserted at the bottom.
4 <-- top

Then 3 is inserted at the bottom
4 <-- top
3

Then 2 is inserted at the bottom
4 <-- top
3
2

Then 1 is inserted at the bottom
4 <-- top
3
2
1
```

So we need a function that inserts at the bottom of a stack using the above given basic stack function.

void insertAtBottom((): First pops all stack items and stores the popped item in function call stack using recursion. And when stack becomes empty, pushes new item and all items stored in call stack.

void reverse(): This function mainly uses insertAtBottom() to pop all items one by one and insert the popped items at the bottom.

C Programming:

``````// C program to reverse a stack using recursion
#include<stdio.h>
#include<stdlib.h>
#define bool int

/* structure of a stack node */
struct sNode
{
char data;
struct sNode *next;
};

/* Function Prototypes */
void push(struct sNode** top_ref, int new_data);
int pop(struct sNode** top_ref);
bool isEmpty(struct sNode* top);
void print(struct sNode* top);

// Below is a recursive function that inserts an element
// at the bottom of a stack.
void insertAtBottom(struct sNode** top_ref, int item)
{
if (isEmpty(*top_ref))
push(top_ref, item);
else
{

/* Hold all items in Function Call Stack until we
reach end of the stack. When the stack becomes
empty, the isEmpty(*top_ref)becomes true, the
above if part is executed and the item is inserted
at the bottom */
int temp = pop(top_ref);
insertAtBottom(top_ref, item);

/* Once the item is inserted at the bottom, push all
the items held in Function Call Stack */
push(top_ref, temp);
}
}

// Below is the function that reverses the given stack using
// insertAtBottom()
void reverse(struct sNode** top_ref)
{
if (!isEmpty(*top_ref))
{
/* Hold all items in Function Call Stack until we
reach end of the stack */
int temp = pop(top_ref);
reverse(top_ref);

/* Insert all the items (held in Function Call Stack)
one by one from the bottom to top. Every item is
inserted at the bottom */
insertAtBottom(top_ref, temp);
}
}

/* Driveer program to test above functions */
int main()
{
struct sNode *s = NULL;
push(&s, 4);
push(&s, 3);
push(&s, 2);
push(&s, 1);

printf("\n Original Stack ");
print(s);
reverse(&s);
printf("\n Reversed Stack ");
print(s);
return 0;
}

/* Function to check if the stack is empty */
bool isEmpty(struct sNode* top)
{
return (top == NULL)? 1 : 0;
}

/* Function to push an item to stack*/
void push(struct sNode** top_ref, int new_data)
{
/* allocate node */
struct sNode* new_node =
(struct sNode*) malloc(sizeof(struct sNode));

if (new_node == NULL)
{
printf("Stack overflow \n");
exit(0);
}

/* put in the data  */
new_node->data  = new_data;

/* link the old list off the new node */
new_node->next = (*top_ref);

/* move the head to point to the new node */
(*top_ref)    = new_node;
}

/* Function to pop an item from stack*/
int pop(struct sNode** top_ref)
{
char res;
struct sNode *top;

/*If stack is empty then error */
if (*top_ref == NULL)
{
printf("Stack overflow \n");
exit(0);
}
else
{
top = *top_ref;
res = top->data;
*top_ref = top->next;
free(top);
return res;
}
}

/* Functrion to pront a linked list */
void print(struct sNode* top)
{
printf("\n");
while (top != NULL)
{
printf(" %d ", top->data);
top =  top->next;
}
}``````

Output:

``` Original Stack
1  2  3  4
Reversed Stack
4  3  2  1```
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#### Venkatesan Prabu

Wikitechy Founder, Author, International Speaker, and Job Consultant. My role as the CEO of Wikitechy, I help businesses build their next generation digital platforms and help with their product innovation and growth strategy. I'm a frequent speaker at tech conferences and events.