使用更大和向量声明优先级队列

Question

Can anybody please explain what is the meaning of this declaration:

typedef pair<long long, int> PII;
priority_queue<PII, vector<PII>, greater<PII> > Q;

Is it to be treated as a priority queue of pair of long long and int or something else? Can please someone also explain what is greater<PII> here?

Answer 1

This declares a std::priority_queue containing std::pair<long long, int> instances where a std::vector<std::pair<long long, int>> is the underlying container of the priority queue (as it is a container adaptor).

The std::greater<std::pair<long long, int>> is used as the Comparator function object for the queue, checking whether the LHS pair is greater than the RHS pair. See below for a reference:

http://en.cppreference.com/w/cpp/utility/functional/greater

Answer 2

Using greater here means that lower values are considered of higher priority and come out of the priority queue earlier. By default less is used, and higher values are higher priority.

Pairs are ordered lexicographically; (1, 3) comes before (2, 1) and after (1, 2).

Answer 3

This is way to create min heap in c++ .

typedef pair<long long, int> PII;
priority_queue<PII, vector<PII>, greater<PII> > Q;