🔄 循环队列 工作原理

容量 MAXN = 8  ·  牺牲 1 个空间判满
MAXN
= 8
head = 0
tail = 0
📏 大小 = 0
状态: ⭕ 空队列
❶ 入队(enqueue)
tail = (tail + 1) % MAXN
tail = (0 + 1) % 8 = 1
q[tail] = x
❷ 出队(dequeue)
head = (head + 1) % MAXN
head = (0 + 1) % 8 = 1
❸ 队空判断
head == tail
0 == 0 → ✅ 队空
❹ 队满判断(牺牲 1 空间)
(tail + 1) % MAXN == head
(0 + 1) % 8 == 0 → ❌ 未满
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