heapq documentation: https://docs.python.org/3.11/library/heapq.html It contains the following line: This implementation uses arrays for which heap[k] <= heap[2*k+1] and heap[k] <= heap[2*k+2] for all *k*, counting elements from zero. heap[k] >= heap[2*k+1] If like to apologise in advance if this is wrong and a waste of your precious time. ☺️