题型分析

题型	分值	频率	难度
Enqueue with circular wrap-around	6 marks	High	★★★
Dequeue with circular wrap-around	6 marks	High	★★★
Full / empty state detection	2–3 marks	Medium	★★

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Q1: Enqueue (6 marks)

Identify: Prompt says "add an item to the circular queue" or "enqueue with wrap-around".

Method:

1. Check if queue is full
2. Update TailPointer with wrap
3. Insert item
4. Increment NumberOfItems

MS Pattern:

M1 Check if full (1 mark)

• IF NumberOfItems = QueueSize THEN or equivalent

A1 Wrap TailPointer (1 mark)

• TailPointer ← (TailPointer + 1) MOD QueueSize
• Must use MOD, not manual IF

A1 Insert item (1 mark)

• Queue[TailPointer] ← NewItem

A1 Increment NumberOfItems (1 mark)

• NumberOfItems ← NumberOfItems + 1

A1 Output full message (1 mark)

• Appropriate message when NumberOfItems == QueueSize

A1 Correct ENDIF, variable types (1 mark)

• Proper structure and typed parameters

Traps:

• ❌ Using TailPointer ← TailPointer + 1 without MOD — only gets M1, loses A1
• ❌ Checking IF TailPointer == QueueSize THEN TailPointer = 0 — not accepted, must use MOD
• ❌ Forgetting to increment NumberOfItems
• ❌ Inserting item before wrapping TailPointer

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Q2: Dequeue (6 marks)

Identify: Prompt says "remove an item from the circular queue" or "dequeue with wrap-around".

Method:

1. Check if queue is empty
2. Wrap HeadPointer
3. Retrieve item
4. Decrement NumberOfItems
5. Return item

MS Pattern:

M1 Check if empty (1 mark)

• IF NumberOfItems = 0 THEN or equivalent

A1 Wrap HeadPointer (1 mark)

• HeadPointer ← (HeadPointer + 1) MOD QueueSize

A1 Retrieve item (1 mark)

• Item ← Queue[HeadPointer]

A1 Decrement NumberOfItems (1 mark)

• NumberOfItems ← NumberOfItems - 1
• Must be after retrieval

A1 Return item (1 mark)

• RETURN Item or equivalent output

A1 Output empty message (1 mark)

• Appropriate message when NumberOfItems == 0

Traps:

• ❌ Wrapping HeadPointer before checking empty — algorithm breaks
• ❌ Returning item before decrementing — NumberOfItems out of sync
• ❌ Using HeadPointer - 1 or wrong MOD direction

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真题示例

Exam	Paper	Question	Type
9618/s25/qp/41	Paper 4	Q1	Enqueue + Dequeue
9618/s23/qp/42	Paper 4	Q2	Enqueue with wrap

复习建议

For each past paper, practice writing the full procedure from memory before checking the mark scheme. Pay special attention to the MOD operation — it is the single most common place to lose marks.