Question

For starters this is apart of my current homework assignment for my Data Structures class. I am not asking for answers, I am asking for help.

I have a stack class that is implementing a Linked List instead of an array. I am currently trying to write my pop() function. I have a node for my theBack part of the stack.

I am just confused as to getting to the predecesor of my theBack node.

Any help with this would be awesome! Thanks!

Answer 1

A stack is actually reasonably simple to implement as a singly linked list, due to its restricted push and pop operations. It’s actually a lot easier if you insert pushed elements at the head of the list. Since it’s homework, I’ll provide pseudo-code.

To initialise a stack, it’s simply creating:

top -> null

with this code:

def init (stk):
    stk->top = null                    # just initialise to empty.

Pushing an item is actually inserting it at the start of the list. So, when you push 3, 4 and 5, you get:

       +---+
top -> | 3 | -> null
       +---+
       +---+    +---+
top -> | 4 | -> | 3 | -> null
       +---+    +---+
       +---+    +---+    +---+
top -> | 5 | -> | 4 | -> | 3 | -> null
       +---+    +---+    +---+

with the following code:

def push (stk, val):
    item = new node                     # Create the node.
    item->value = val                   # Insert value.
    item->next = stk->top               # Point it at current top.
    stk->top = item                     # Change top pointer to point to it.

And popping is then just removing that first node.

def pop (stk):
    if stk->top == null:                # Catch stack empty error.
        abort with "stack empty"
    first = stk->top                    # Save it for freeing.
    val = first->value                  # Get the value from the top.
    stk->top = first->next              # Set top to the top's next.
    free first                          # Release the memory.
    return val                          # Return the value.