考前速记

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关键公式

Enqueue:  TailPointer = (TailPointer + 1) MOD QueueSize
Dequeue: HeadPointer = (HeadPointer + 1) MOD QueueSize

---

初始化

HeadPointer = -1
TailPointer = -1
NumberOfItems = 0
Queue = [None] * QueueSize

---

Enqueue — 4行核心

IF NumberOfItems < QueueSize THEN
    TailPointer = (TailPointer + 1) MOD QueueSize
    Queue[TailPointer] = NewItem
    NumberOfItems = NumberOfItems + 1

---

Dequeue — 4行核心

IF NumberOfItems > 0 THEN
    HeadPointer = (HeadPointer + 1) MOD QueueSize
    Item = Queue[HeadPointer]
    NumberOfItems = NumberOfItems - 1
    RETURN Item

---

Full / Empty

State	Check	Action
Full	NumberOfItems == QueueSize	Reject enqueue
Empty	NumberOfItems == 0	Reject dequeue

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高频陷阱

• ❌ Manual IF wrapping → always use MOD
• ❌ Pointer compare for full/empty → use NumberOfItems
• ❌ Decrement before retrieval → retrieve, then decrement
• ❌ No return in empty branch → return Null or ""

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伪代码速查模板

// --- Enqueue ---
PROCEDURE Enqueue(Item)
    IF NumberOfItems < MaxSize THEN
        TailPointer = (TailPointer + 1) MOD MaxSize
        Queue[TailPointer] = Item
        NumberOfItems = NumberOfItems + 1
    ELSE
        OUTPUT "Queue full"
    ENDIF
ENDPROCEDURE

// --- Dequeue ---
FUNCTION Dequeue() RETURNS STRING
    IF NumberOfItems > 0 THEN
        HeadPointer = (HeadPointer + 1) MOD MaxSize
        Item = Queue[HeadPointer]
        NumberOfItems = NumberOfItems - 1
        RETURN Item
    ELSE
        OUTPUT "Queue empty"
        RETURN ""
    ENDIF
ENDFUNCTION

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最后提醒

• M1 is always the full/empty check — worth 1 mark, do not skip it
• MOD is worth 1 mark — the easiest mark to lose
• Write pseudocode, not Python, unless the question asks for Python