Question

there’s something I can’t get my head round…
Basically I’m given the following data structure:

struct node_ll {
    int payload;
    node_ll *next;  //pointer to next node
};

Which is essentially a stack of numbers.
I need to create a method with the following prototype:

int tail_return(node_ll **list)

where **list is the memory address of the above data structure. My implementation is as follows:

int tail_return(node_ll **list) {

    node_ll *temp;
    temp = *list;

    node_ll *prev_temp;
    prev_temp = *list;

    bool firstPass = true;

    while(temp){

        if(firstPass == true){
        temp = temp->next;
        firstPass = false;

        } else {
            temp = temp->next;
            prev_temp = prev_temp->next;

        }
    }

    int toReturn = prev_temp->payload;

    prev_temp->payload = 0;
    (**list).next = prev_temp;

    delete temp;
    delete prev_temp;
    return toReturn;
}

However I get the following output from test runs:

List a after head insertion of 2,4,6,8,10 elements:

{10,8,6,4,2}

now removing the last element

DELETED: 2

{10,0} where it's supposed to be: {10,8,6,4}

What am I doing wrong? Apparently the method finds the right value to delete – 2. But why when I try to print it after deletion I end up with 10 and 0?

Answer 1

 (**list).next = prev_temp;

should be

prev_temp->next = 0 ; 

when you do (**list).next = prev_temp; you are manipulating the parameter which was passed to your method and not the last node in the linked list.