comparator priority queue c++

To use a comparator with a priority queue in C++, you can follow these steps:

  1. Include the necessary header files: At the beginning of your program, include the <queue> and <functional> header files to use the priority queue and the comparator respectively.

  2. Define the comparator function or class: Next, define a function or class that will act as the comparator. This function or class should have a bool return type and take two parameters of the same type as the priority queue elements. The function or class should compare the two elements and return true if the first element should have a higher priority than the second element, and false otherwise.

  3. Create a priority queue object with the comparator: Declare a priority queue object and specify the type of elements it will hold, along with the comparator. You can do this by using the following syntax: priority_queue<T, vector<T>, Comparator> pq;. Replace T with the type of elements you want to store and Comparator with the name of your comparator function or class.

  4. Use the priority queue: You can now use the priority queue as usual, by inserting elements using the push() function and accessing the top element using the top() function. The elements will be ordered according to the rules defined in your comparator function or class.

Here's an example that demonstrates the steps mentioned above:

#include <queue>
#include <functional>

// Step 2: Define the comparator function or class
struct MyComparator {
    bool operator()(int a, int b) {
        // Compare the elements in reverse order
        return a > b;
    }
};

int main() {
    // Step 3: Create a priority queue object with the comparator
    std::priority_queue<int, std::vector<int>, MyComparator> pq;

    // Step 4: Use the priority queue
    pq.push(5);
    pq.push(3);
    pq.push(7);

    while (!pq.empty()) {
        std::cout << pq.top() << " ";
        pq.pop();
    }

    return 0;
}

In the example above, the priority queue will order the elements in descending order, as specified by the MyComparator structure. The output will be: 7 5 3.