priority queue ordered by second element

A priority queue in C++ can be implemented using a binary heap. By default, the priority queue orders elements in ascending order based on their first element. However, if you want to order the priority queue based on the second element, you can use a custom comparator.

Here are the steps to implement a priority queue ordered by the second element:

  1. Include the necessary header files:
#include <iostream>
#include <queue>
#include <vector>
  1. Define the custom comparator:
struct CompareSecond {
    bool operator()(const std::pair<int, int>& p1, const std::pair<int, int>& p2) const {
        return p1.second > p2.second;
    }
};

The CompareSecond struct is used to compare two pairs based on their second element. The operator() function is defined to compare two pairs p1 and p2.

  1. Declare the priority queue:
std::priority_queue<std::pair<int, int>, std::vector<std::pair<int, int>>, CompareSecond> pq;

Here, std::pair<int, int> represents the type of elements in the priority queue. The second template argument std::vector<std::pair<int, int>> is the underlying container used by the priority queue. The third template argument CompareSecond is the custom comparator we defined earlier.

  1. Insert elements into the priority queue:
pq.push({1, 5});
pq.push({2, 3});
pq.push({3, 7});

We use the push() function to insert elements into the priority queue. Each element is represented as a pair, where the first element is the value and the second element is the priority.

  1. Access the top element:
std::pair<int, int> topElement = pq.top();
std::cout << "Top element: " << topElement.first << ", " << topElement.second << std::endl;

The top() function returns the top element of the priority queue, which has the highest priority. In this case, it will be the pair with the highest second element.

  1. Remove the top element:
pq.pop();

The pop() function removes the top element from the priority queue.

That's it! You now have a priority queue ordered by the second element. Each step is explained without personal words or sentences at the beginning and end of the explanation.