How to remove element not at top from priority_queue?

Question

In my program I need to delete an element from a priority queue that is not at the top. Can that be done? If not, please suggest a way to do so except creating your own heap.

Answer 1

The standard priority_queue<T> can be customized through inheritance. It has protected members c and comp that can be referenced in a descendant class.

template<typename T>
class custom_priority_queue : public std::priority_queue<T, std::vector<T>>
{
  public:

      bool remove(const T& value) {
        auto it = std::find(this->c.begin(), this->c.end(), value);
        if (it != this->c.end()) {
            this->c.erase(it);
            std::make_heap(this->c.begin(), this->c.end(), this->comp);
            return true;
       }
       else {
        return false;
       }
 }
};

void main()
{
   custom_priority_queue<int> queue;

   queue.push(10);
   queue.push(2);
   queue.push(4);
   queue.push(6);
   queue.push(3);

   queue.remove(6);

   while (!queue.empty())
   {
      std::cout << queue.top();
      queue.pop();

      if (!queue.empty())
      {
        std::cout << ", ";
      }
   }

 }

Output:

10, 4, 3, 2

Answer 2

Pradip and MASh sacrifice the time to realize the remove operation. But if time complexity is important to you, I suggest you to use hash min_heap. A Hash table stores the value-pointer and the pointers point to a min_heap. Which means you can spend O(1) time to find the value in min_heap and O(log(n)) to remove(sift-up or sift down) the element.

Answer 3

The best solution is to use std::set. Sets provide methods which allow it to be used both as a min/max heap (or a priority queue).

std::set<int> pq;

//accessing the smallest element(use as min heap)
*pq.begin();

//accessing the largest element (use as max heap)
*pq.rbegin();

Furthermore sets also allow random deletion.

//to delete the integer '6'
auto it = pq.find(6);
pq.erase(it);

Answer 4

Let you want to delete the 5th element in the priority_queue<type> Q . Then you can do this like:

vector<type> tempQ;
int i=0;
int n=5;
type t;
while(i<n-1)
{
    tempQ.push_back(Q.top());
    Q.pop();        
    i++;
}
Q.pop();
i=0;
while(i<n-1)
{
    t=tempQ[i++];
    Q.push(t);
}