realloc():
memory when gets decreased, we can allocate it with the 
function called realloc().
free:
it is used to free the memory in the program.

free()is a macro used to deallocate memory.
when we need more memory realoc() reallocate memory 
according to given size, if there is no memory 4 
alocation ,it will free previous allocated memory & 
allocate new one using malloc().
ex: int *p = realloc(p,sizeof(p)*2);

realloc() can be use to reallocation of the memory if the 
memory already created is insufficient,
but free can be used to destroy the memory which was 
already created