1. This lab is designed to give you practice working with linked lists.
   The pointer variable mylist (in the main function) points to the first element in the linked list. A function make_a_list builds the linked list. Three other functions: print_list, add_at_front and add_at_back, are not complete. The calls to the other functions are alreadty given in the main program (some are commented out). Your assignment is to complete these functions.
2. The source file: lists.cc
3. Understand Each of the List Functions..
   • make_a_list
     This function builds the linked list. Note that head points to the head, or start, of the list, and is initially NULL since the list starts out empty.
     The functions loops through the integers 20 down to 2. During each iteration, a new node is created. This new node will be inserted at the beginning of the list. The value field of this node gets assigned the current integer (num), and the next field of the node is assigned what head used to point to.
     When the loop finishes, the value of head, which points to the first element of the list, automatically goes back to the calling function, since it is itself a reference parameter.
   • print_list
     Once you understand the previous code, a good place to start is the print_list function. Basically, you want to loop through each node in the list until you reach a NULL value (which indicates the end of the list). As you are looping, you want to print the integer value stored in that node.
   • add_at_front
     The add_at_front function will insert a new node at the beginning of the list. So, you want to first create a new node. Next, assign to this new node (in its value field) the integer value (addvalue) that was passed to the function. You also want the node's next field to point to the previous first element of the list. Finally, you want the head of the list to point to this new first element.
   • add_at_back
     In this function, you will need to loop through all elements of the list until you reach the last element of the list. Since we are adding a new node to the list, you will want to create a new node. Then, assign to this new node the integer value that was passed to the function. Since this new node will be the last node in the list, be sure to assign its net field appropriately. Finally, don't forget to have the previous node point to the new node.
4. What you have to do:
   • Notice that each of the add functions have similar code to make the new node. This suggests that a function might be good: create a function, called make_new_node which takes a single int as an argument, builds a new node, and returns the address of the new node. The change the "add" functions so that they use the new function. Once this works, make a script file to hand in.
   • Next, change each "add" function so that is properly handles adding to an empty list ( where head is null ). Make a script with the source code and a run of a small test program to show this working.
   • Finally, the function make_a_list should call the "add" functions to make the list. Have it alternately call add_at_back and add_at_front. Make a script showing this working. Hand in all 3 of the above scripts.