reverse-stack-recursion-c.c

// 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); 
	} 
} 

// Driver Code 
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; 
	} 
} 

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