算法稳定性

最近在看算法，书上关于稳定性的概念一带而过，甚觉不爽，于是网上搜了个貌似靠谱的答案。答案来自stackoverfollow

问：
I m very curious, why stability is or is not important in sorting algorithms?

Any ideas?

靠谱的答：
Background: a "stable" sorting algorithm keeps the items with the same sorting key in order. Suppose we have a list of 5-letter words:

peach straw apple spork

Stable-sorting by the first letter gives us:

apple peach straw spork

In an unstable algorithm, straw or spork may be interchanged, but in stable sort, they stay in the same relative positions (that is, since 'straw' appears before 'spork' in the input, it also appears before 'spork' in the output).

We could sort the list of words using this algorithm: stable sorting by column 5, then 4, then 3, then 2, then 1. In the end, it will be correctly sorted. Convince yourself of that. (by the way, that algorithm is called radix sort)

Now to answer your question, suppose we have a list of first and last names. We are asked to sort "by last name, then by first". We could first stable sort by the first name, then sort by the last name. After these sorts, the list is primarily sorted by the last name. However, where last names are the same, the first names are sorted.

You can't stack unstable sorts in the same fashion