![]() ![]() total = 0 def insert ( self, i, key ): assert type ( i ) is int if i >= self. We also have to modify swim and sink methods to support changing the key of an element inside the queue.Ĭlass IndexedMinPQ : def _init_ ( self, N ): self. ![]() The keys (keys), the heap representation of the priority queue (pq), and an array containing the index of each key in the heap representation (qp). In an Indexed priority queue, we store the information in three arrays. However, with a minor change, we can add indexing to the priority queue. While priority queues are useful in many applications, they do not support changing the priority of an element in logarithmic time. _sink ( 1 ) return out raise Exception ( 'PQ is empty' ) _swim ( len ( arr ) - 1 ) def remove (): if not self. arr idx = p else : break def isEmpty (): return len ( self. arr idx = child else : break def _swim ( idx ): assert type ( idx ) is int assert idx > 0 and idx 0 : p = idx // 2 if self. arr : child = child + 1 if arr > arr : self. N = N def _sink ( self, idx ): assert type ( idx ) is int assert idx > 0 and idx self. ![]() The swim method ensures that the node does not have a higher priority than its parent, and the sink method makes sure that the node’s children have lower priority.Ĭlass PQ : def _init_ ( self, N ): self. Heaps use the swim and the sink methods to ensure that the node with the highest priority is always on top of the tree. To have efficient access to the node’s children or the node’s parent, an array is usually used to represent the tree where if the key of node i is pq, the children are pq and pq. A heap is a tree-based data structure where for node C, C’s key is greater than the key of C’s parent (for MinPQ). Priority queues are usually implemented with heaps. But what if in the middle of the program, the priority of an object changes? Indexed Priority Queue gives us the ability to change the priority of an element without having to go through all the elements.īefore going through the details on how to implement an Indexed Priority Queue, let’s see how we can implement a priority queue. However, instead of removing the ‘First In’ element, the priority queue removes the element with the highest ‘priority’. You could easily wrap this behavior in a small function so that you repeat yourself less.Like a regular queue, the priority queue is an abstract data structure that supports adding and removing elements. If you want to preserve insert order in case of duplicates, you can do this: from queue import PriorityQueueĪnd proceed similarly with heapq. In that case, it depends how you want to deal with duplicate values. If you have duplicate values, then the comparison will look at the second element of the tuple, and will break because your Nodes are not comparable. ![]() With queue.PriorityQueue, use tuples as suggested by the doc from queue import PriorityQueue P.queue # Īnother way, if you can specify the priority of each element as you push it in. Then just add them normally: p = PriorityQueue() Or if you want to just pass Node objects that exist already: ComparableNode: So you can wrap it in a class that you create. Let's assume you have this setup: class Node:īut you can't modify Node. One solution, without passing a comparator, is to wrap your Node objects in another object, say ComparableNode, which implements the comparisons you would like on the Node object. I understand that the cause of this error is that the Node objects need to be comparable to one another, and the way to achieve this according to the documentation is to at least implement lt and eq methods for Node.īut for my problem, since I wont be able to modify the Node class, will I be able to pass a way( a lambda or a Comparator) to the PriorityQueue to help it determine the ordering.(I am expecting something Java like)Īny alternative to achieve this is also appreciated.(remember Node is not modifiable) Q.put((b.val, b)) // Errors TypeError: '<' not supported between instances of 'Node' and 'Node' Q.put(b) // Errors TypeError: '<' not supported between instances of 'Node' and 'Node' It still compares the 'node' object to resolve the ties. It still gives me "TypeError: '<' not supported between instances of 'Node' and 'Node'". I tried adding them as tuples(node.val, node) to the queue. I need these objects prioritized based on a field of the object. I want to use a PriorityQueue to which I need to add objects of a class (say Node) which I cannot modify. ![]()
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