Объясните код списка - C (СИ)

#include <stdlib.h>
#include <stdio.h>
 
#define SIZE 11
 
typedef int TE;
 
struct list
{
    TE data;
    struct list *next;
};
 
struct list* add ( struct list *list0, TE n )
{
    struct list *pointer = malloc( sizeof( struct list ) );
    pointer -> data = n;
    pointer -> next = list0;
    list0 = pointer;
    return list0;
}
 
void print ( struct list *list0 )
{
    struct list *pointer = list0;
    while ( pointer != 0 )
    {
        printf( "%4d", pointer -> data );
        pointer = pointer -> next;
    }
    printf( "\n" );
}
 
void printToFile( FILE *f, struct list *list0 )
{
    struct list *pointer = list0;
    while ( pointer != 0 )
    {
        fprintf( f, "%4d", pointer -> data );
        pointer = pointer -> next;
    }
    fprintf( f, "\n" );
}
 
unsigned int is_empty( struct list * list0 )
{
    return list0 == NULL;
}
 
struct list* insert( struct list *list0, TE n )
{
    struct list *p = malloc( sizeof( struct list ) ), *cur;
    p -> data = n;
    p -> next = 0;
    if ( !list0 )
        list0 = p;
    else
    {
        cur = list0;
        while ( cur -> next != 0 )
            cur = cur -> next;
        p -> next = cur -> next;
            cur -> next = p;
    }
    return list0;
}
 
float average( struct list *list0 )
{
    struct list *pointer = list0;
    float sum = 0.0;
    unsigned int number = 0;
 
    while ( pointer )
    {
        number++;
        sum += pointer -> data;
        pointer = pointer -> next;
    }
 
    return sum * 1.0 / number;
}
 
struct list* swapFrontBack( struct list *list0 )
{
    struct list *pointer = list0;
    while ( pointer -> next )
        pointer = pointer -> next;
 
    int temp = list0 -> data;
    list0 -> data = pointer -> data;
    pointer -> data = temp;
 
    return list0;
}
 
struct list* replace( struct list *list0, TE e1, TE e2 )
{
    struct list *pointer = list0;
    while ( pointer )
    {
        if ( pointer -> data == e1 )
            pointer -> data = e2;
        pointer = pointer -> next;
    }
    return list0;
}
 
int main()
{
    struct list *head = NULL;
 
    int index;
    for( index = 0; index <= SIZE; ++index )
    {
        head = insert( head, index );
    }
 
    head = swapFrontBack( head );
    replace( head, 4, 111 );
    print( head );
 
    printf( "Average = %7.4f\n", average( head ) );
 
    FILE *file = fopen( "input1.txt", "w" );
    printToFile( file, head );
    fclose( file );
 
    return 0;
}