Increase priority in priority queue

Question

I want to decrease the priority of an element in a priority queue? How do I achieve that in logarithmic time?

I know priority queues are implemented as max heaps. But if I modify any element in the queue and then call make_heap on it, it will take linear time in the number of elements.

Reference: http://www.cplusplus.com/reference/queue/priority_queue/

Answer 1

The only way to perform updating with logarithmic complexity is to implement the priority queue yourself, so you can use well-known algorithms .

The implementation of std::priority_queue is not specified by the standard (for example it can well be d-ary heap ), so you can't portably update the priority without rebuilding the queue. For the same reason you can't use std::make_heap and related functions.

Answer 2

You cannot do it using std::priority_queue . However, it is possible if you have an access to the internal structure of the heap. If you have a pointer to the element, you can decrease the priority of an element and then use siftDown procedure for this element only to fix the heap.