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增加时间复杂度的描述

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Original file line number	Diff line number	Diff line change
`@@ -250,7 +250,7 @@ static class Segment<K,V> extends ReentrantLock implements Serializable {`
`250`	`250`
`251`	`251`	`### JDK1.8 (上面有示意图)`
`252`	`252`
`253`		`-ConcurrentHashMap取消了Segment分段锁,采用CAS和synchronized来保证并发安全。数据结构跟HashMap1.8的结构类似,数组+链表/红黑二叉树。`
	`253`	`+ConcurrentHashMap取消了Segment分段锁,采用CAS和synchronized来保证并发安全。数据结构跟HashMap1.8的结构类似,数组+链表/红黑二叉树。Java 8在链表长度超过一定阈值(8)时将链表(寻址时间复杂度为O(N))转换为红黑树(寻址时间复杂度为O(long(N)))`
`254`	`254`
`255`	`255`	`synchronized只锁定当前链表或红黑二叉树的首节点,这样只要hash不冲突,就不会产生并发,效率又提升N倍。`
`256`	`256`

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