Delete Node at End in DSA C - Execution Trace

Concept Flow - Delete Node at End

Start at head

↓

Check if list empty?

Yes→Nothing to delete, EXIT

No↓

Traverse to second last node

↓

Remove last node

↓

Update second last node's next to NULL

↓

Free last node memory

↓

End

Start from the head, check if list is empty, then move to the second last node, remove the last node by updating pointers and freeing memory.

Execution Sample

DSA C

typedef struct Node {
    int data;
    struct Node* next;
} Node;

void deleteAtEnd(Node** head) {
    if (*head == NULL) return;
    if ((*head)->next == NULL) {
        free(*head);
        *head = NULL;
        return;
    }
    Node* temp = *head;
    while (temp->next->next != NULL) temp = temp->next;
    free(temp->next);
    temp->next = NULL;
}

Deletes the last node of a singly linked list by traversing to the second last node and freeing the last node.

Execution Table

Step	Action	Pointer Positions	Condition	Result
1	Check if head is NULL	head -> Node1	head == NULL? No	Continue
2	Check if head->next is NULL	head -> Node1	head->next == NULL? No	Continue
3	Set temp = head	temp -> Node1	-	temp points to Node1
4	Check temp->next->next != NULL	temp -> Node1	Node2->next != NULL? Yes	Move temp to next
5	Move temp to next	temp -> Node2	-	temp points to Node2
6	Check temp->next->next != NULL	temp -> Node2	Node3->next != NULL? No	Stop loop
7	Free temp->next (Node3)	temp -> Node2	-	Node3 freed
8	Set temp->next = NULL	temp -> Node2	-	Node2->next = NULL
9	End	head -> Node1	-	Last node deleted, list ends at Node2

💡 Loop ends when temp->next->next is NULL, meaning temp is second last node.

Variable Tracker

Variable	Start	After Step 3	After Step 5	After Step 6	Final
head	Node1 -> Node2 -> Node3 -> NULL	Node1 -> Node2 -> Node3 -> NULL	Node1 -> Node2 -> Node3 -> NULL	Node1 -> Node2 -> Node3 -> NULL	Node1 -> Node2 -> NULL
temp	NULL	Node1	Node2	Node2	Node2

Key Moments - 3 Insights

Why do we check temp->next->next != NULL instead of temp->next != NULL in the loop?

Because we want temp to stop at the second last node, so that temp->next is the last node to delete. This is shown in execution_table rows 4-6.

What happens if the list has only one node?

The code frees the head node and sets head to NULL immediately (rows 1-2), avoiding traversal.

Why do we set temp->next = NULL after freeing the last node?

To update the second last node's next pointer to NULL, marking the new end of the list (row 8).

Visual Quiz - 3 Questions

Look at the execution_table, what node does temp point to after Step 5?

A Node1

B Node2

C Node3

D NULL

💡 Hint

Check the 'Pointer Positions' column at Step 5 in execution_table.

At which step does the loop stop traversing the list?

A Step 6

B Step 5

C Step 4

D Step 7

💡 Hint

Look for the step where condition temp->next->next != NULL becomes false in execution_table.

If the list had only one node, which step would free that node?

A Step 1

B Step 7

C Step 2

D Step 8

💡 Hint

Refer to the early check for single node list in execution_table rows 1-2.

Concept Snapshot

Delete Node at End in singly linked list:
- If list empty, do nothing.
- If only one node, free it and set head NULL.
- Else, traverse to second last node.
- Free last node and set second last's next to NULL.
- Updates list end safely.

Full Transcript

This visual execution traces deleting the last node in a singly linked list. It starts by checking if the list is empty or has only one node. If one node, it frees it and sets head to NULL. Otherwise, it traverses to the second last node by checking temp->next->next != NULL. Once at second last node, it frees the last node and sets second last's next pointer to NULL, marking the new end of the list. Variables head and temp change as shown. Key moments include why we check temp->next->next, handling single node list, and updating pointers after deletion. The quiz tests understanding of pointer positions and loop stopping conditions.