heapq

heapq是python自带的堆+优先级队列

官网https://docs.python.org/3.7/library/heapq.html

要点1：heapq实现的是小顶堆

要点2：大顶堆的做法是： [-i for i in heap]

下面是它的类函数

heapq.heappush(heap,item) 放入堆

Push the value item onto the heap, maintaining the heap invariant.

heapq.heappop(heap) 取出最小一个元素

Pop and return the smallest item from the heap, maintaining the heap invariant. If the heap is empty, IndexError is raised. To access the smallest item without popping it, use heap[0].

heapq.heappushpop(heap,item) 前面两个操作合体

Push item on the heap, then pop and return the smallest item from the heap. The combined action runs more efficiently than heappush() followed by a separate call to heappop().

heapq.heapify(x) 堆化（列表或者map）

Transform list x into a heap, in-place, in linear time.

heapq.heapreplace(heap,item) //注意比较和pushpop的区别。元素值在任意时刻固定

Pop and return the smallest item from the heap, and also push the new item. The heap size doesn’t change. If the heap is empty, IndexError is raised.

This one step operation is more efficient than a heappop() followed by heappush() and can be more appropriate when using a fixed-size heap. The pop/push combination always returns an element from the heap and replaces it with item.

The value returned may be larger than the item added. If that isn’t desired, consider using heappushpop()instead. Its push/pop combination returns the smaller of the two values, leaving the larger value on the heap.

The module also offers three general purpose functions based on heaps.

heapq.merge(*iterables,key=None,reverse=False)

Merge multiple sorted inputs into a single sorted output (for example, merge timestamped entries from multiple log files). Returns an iterator over the sorted values.

Similar to sorted(itertools.chain(*iterables)) but returns an iterable, does not pull the data into memory all at once, and assumes that each of the input streams is already sorted (smallest to largest).

Has two optional arguments which must be specified as keyword arguments.

key specifies a key function of one argument that is used to extract a comparison key from each input element. The default value is None (compare the elements directly).

reverse is a boolean value. If set to True, then the input elements are merged as if each comparison were reversed. To achieve behavior similar to sorted(itertools.chain(*iterables), reverse=True), all iterables must be sorted from largest to smallest.

Changed in version 3.5: Added the optional key and reverse parameters.

heapq.nlargest(n,iterable,key=None) topk最大

Return a list with the n largest elements from the dataset defined by iterable. key, if provided, specifies a function of one argument that is used to extract a comparison key from each element in iterable (for example, key=str.lower). Equivalent to: sorted(iterable, key=key, reverse=True)[:n].

heapq.nsmallest(n,iterable,key=None) topk最小

Return a list with the n smallest elements from the dataset defined by iterable. key, if provided, specifies a function of one argument that is used to extract a comparison key from each element in iterable (for example, key=str.lower). Equivalent to: sorted(iterable, key=key)[:n].

The latter two functions perform best for smaller values of n. For larger values, it is more efficient to use the sorted() function. Also, when n==1, it is more efficient to use the built-in min() and max() functions. If repeated usage of these functions is required, consider turning the iterable into an actual heap.