下列字典的平均情况基于以下假设：

1. 对象的散列函数足够撸棒(robust)，不会发生冲突。

2. 字典的键是从所有可能的键的集合中随机选择的。

小窍门：只使用字符串作为字典的键。这么做虽然不会影响算法的时间复杂度，但会对常数项产生显著的影响，这决定了你的一段程序能多快跑完。

操作

平均情况

最坏情况

复制[注2]

O(n)

O(n)

取元素

O(1)

O(n)

更改元素[注1]

O(1)

O(n)

删除元素

O(1)

O(n)

遍历

O(n)

O(n)

注：

[1] = These operations rely on the “Amortized” part of “Amortized Worst Case”. Individual actions may take surprisingly long, depending on the history of the Container.

[2] = For these operations, the worst case n is the maximum size the container ever achieved, rather than just the current size. For example, if N objects are added to a dictionary, then N-1 are deleted, the dictionary will still be sized for N objects (at least) until another insertion is made.

本文转载自：https://www.cnblogs.com/harvey888/p/6659061.html