QList需要使用O(n)的时间去搜索其内容，QSet采用hash技术，仅用O(1)的时间复杂度就可以完成内容的搜索。
Searching through an unsorted sequential container such as QList requires examing every single element and results in a runtime complexity O(n). On the other hand, QSet is a hash-table based container. Unlike sequential containers, sets do not keep elements in the order they were inserted. This has some advantages. For example, containment testing is faster for sets with potential O(1) complexity, as we do not have to perform a linear search.
Sets have this capability thanks to a technique called hashing. A hash function computes an integer value (hash code) from an object. A hash function does not necessarily produce a unique hash code for every object. Normally, it maps a key to a bucket that may contain one or more elements (the number of these buckets can be exposed with QSet::capacity()). The container then iterates over the contents of the bucket until it finds a match. In the best case, with either no collisions and all buckets either empty or with a single element, checking for containment takes constant O(1) time.
QList类似于std::vector
QSet类似于std::set
Reference
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