Indexed Priority Queue in C++
Basics
Priority queues are a useful data structure e.g. when you repeatedly need to access the largest or smallest element of a set of values but want more efficiency than keeping an array ordered. The C++ standard library contains std::priority_queue, which works well for basic use cases. It offers insertion of elements as well as access and removal of the minimum (or maximum). It does not, however, offer a method for changing the priority of a value, which is needed for some applications (like Dijkstra’s algorithm to find shortest paths in graphs). Often such a method is called decreaseKey or changeKey. This post offers an implementation using a binary heap which supports this operation in O(log(n)) time.