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常见错误

1. Infinite Loop in Traversal​

// WRONG — missing pointer advancement
Current ← Head
WHILE Current <> -1
    OUTPUT Node[Current].Data
    // Missing: Current ← Node[Current].NextPointer
ENDWHILE

Fix: Always include Current ← Node[Current].NextPointer as the last line of the loop body.

2. Modifying Head During Traversal​

// WRONG — destroys the head reference
WHILE Head <> -1
    OUTPUT Node[Head].Data
    Head ← Node[Head].NextPointer
ENDWHILE

Fix: Use a working variable Current ← Head; never reassign Head unless intentionally changing the list's start.

3. Forgetting to Update Free Pointer​

// WRONG — free pointer never advances
NewNode ← Free
// Missing: Free ← Node[Free].NextPointer
Node[NewNode].Data ← Value

Fix: Immediately after NewNode ← Free, update Free ← Node[Free].NextPointer.

4. Not Handling Empty List​

// WRONG — assumes Tail always exists
Node[Tail].NextPointer ← NewNode

When the list is empty (Head = -1), Tail is also -1, so Node[Tail] is an invalid index.

Fix: Check IF Head = -1 THEN before accessing Node[Tail].

5. Setting Wrong NextPointer for New Node​

// WRONG for insert-at-end
Node[NewNode].NextPointer ← Head

For end insertion, Node[NewNode].NextPointer must be -1.

Fix: Node[NewNode].NextPointer ← -1 (end insertion) or Node[NewNode].NextPointer ← Head (start insertion).

6. Confusing Index with Data​

OUTPUT Node[Current].NextPointer  // WRONG — outputs pointer, not data
OUTPUT Current                     // WRONG — outputs index, not data

Fix: OUTPUT Node[Current].Data is what the question asks for.

7. Off-by-One in Deletion Loop​

WHILE Current <> -1
    IF Node[Current].Data = Value THEN
        // delete
    ENDIF
    Current ← Node[Current].NextPointer
ENDWHILE

This works but is inefficient — the better pattern searches for the predecessor so you can relink without a second pass.

8. Not Returning Deleted Nodes to Free List​

After deletion, the removed node's index should be prepended to the free list:

Node[Current].NextPointer ← Free
Free ← Current

9. Confusing Tail Update After Deletion​

If the last node is deleted, Tail must be set to the new last node (or -1 if list becomes empty).

10. Parameter Passing Errors​

When passing the linked list to a procedure that modifies it, pass by reference (or use global variables). In pseudocode, this usually means operating on a global array Node[] and global pointers Head, Tail, Free.

Memory Aid: DEBUG checklist
  • Data — did I store/read Data and not the pointer?
  • Empty — did I handle the empty-list case?
  • Bounds — am I accessing a valid array index?
  • Update — did I advance both the working pointer AND Free?
  • Guard — does my loop condition prevent infinite looping?