Reversing a string using stack

/*
Write a program to reverse a string using stack
*/

#include<stdio.h>
#include<string.h>
#include<stdlib.h>

#define MAX 20

int top = -1;
char stack[MAX];
char pop();
void push(char);

main()
{
char str[20];
unsigned int i;
printf("Enter the string : " );
gets(str);

/*Push characters of the string str on the stack */

for(i=0;i<strlen(str);i++)

push(str[i]);

/*Pop characters from the stack and store in string str */

for(i=0;i<strlen(str);i++)

str[i]=pop();

printf("Reversed string is : ");

puts(str);

}

void push(char item)
{

if(top == (MAX-1))

{

printf("Stack Overflow\n");

return;

}

stack[++top] =item;

}

char pop()
{

if(top == -1)

{

printf("Stack Underflow\n");

exit(1);

}

return stack[top--];

}