Python之heapq模块的使用

heapq模块的作用

    堆是一个树形的数据结构,其中子节点与父节点是一种有序关系。二叉堆:可以使用一个有组织的列表或数据表示,其中元素N的子元素位于2*N+1和2*N+2(索引从0开始)。这种布局允许原地重新组织堆,
从而不必在增加或删除元素时重新分配大量内存。

最大堆:确认父节点大于或等于两个子节点。
最小堆:要求父节点大于或等于子节点。
Python的heapq模块实现了一个最大堆。

1、准备演示的数据

data = [19, 9, 4, 10, 11]
heapq_heapdata.py

2、准备演示的显示数据

import math
from io import StringIO


def show_tree(tree, total_width=36, fill=' '):
    """Pretty-print a tree."""
    output = StringIO()
    last_row = -1
    for i, n in enumerate(tree):
        if i:
            row = int(math.floor(math.log(i + 1, 2)))
        else:
            row = 0
        if row != last_row:
            output.write('\n')
        columns = 2 ** row
        col_width = int(math.floor(total_width / columns))
        output.write(str(n).center(col_width, fill))
        last_row = row
    print(output.getvalue())
    print('-' * total_width)
    print()
heapq_showtree.py

3、创建堆(第一种方式:heappush),从数据源增加新元素时会保持元素的堆排序顺序

import heapq
from heapq_showtree import show_tree
from heapq_heapdata import data

heap = []
print('random :', data)
print()

for n in data:
    print('add {:>3}:'.format(n))
    heapq.heappush(heap, n)
    show_tree(heap)
heapq_heappush.py

运行效果

random : [19, 9, 4, 10, 11]

add  19:

                 19                 
------------------------------------

add   9:

                 9                  
        19        
------------------------------------

add   4:

                 4                  
        19                9         
------------------------------------

add  10:

                 4                  
        10                9         
    19   
------------------------------------

add  11:

                 4                  
        10                9         
    19       11   
------------------------------------

4、创建堆(第二种方式:heapify),如果数据已经在内存中,那么使用heapify()原地重新组织列表中的元素会更高效

import heapq
from heapq_showtree import show_tree
from heapq_heapdata import data

print('random    :', data)
heapq.heapify(data)
print('heapified :')
show_tree(data)
heapq_heapify.py

运行效果

random    : [19, 9, 4, 10, 11]
heapified :

                 4                  
        9                 19        
    10       11   
------------------------------------

5、删除最小的元素,heappop()

import heapq
from heapq_showtree import show_tree
from heapq_heapdata import data

print('random    :', data)
heapq.heapify(data)
print('heapified :')
show_tree(data)
print()

for i in range(2):
    smallest = heapq.heappop(data)
    print('pop    {:>3}:'.format(smallest))
    show_tree(data)
heapq_heappop.py
random    : [19, 9, 4, 10, 11]
heapified :

                 4                  
        9                 19        
    10       11   
------------------------------------


pop      4:

                 9                  
        10                19        
    11   
------------------------------------

pop      9:

                 10                 
        11                19        
------------------------------------

6、删除现有的值,并且增加新元素替换

import heapq
from heapq_showtree import show_tree
from heapq_heapdata import data

heapq.heapify(data)
print('start:')
show_tree(data)

for n in [0, 13]:
    smallest = heapq.heapreplace(data, n)
    print('replace {:>2} with {:>2}:'.format(smallest, n))
    show_tree(data)
heapq_heapreplace.py

 运行效果

start:

                 4                  
        9                 19        
    10       11   
------------------------------------

replace  4 with  0:

                 0                  
        9                 19        
    10       11   
------------------------------------

replace  0 with 13:

                 9                  
        10                19        
    13       11   
------------------------------------

 7、堆数据的极限值

import heapq
from heapq_heapdata import data

print('all       :', data)
print('3 largest :', heapq.nlargest(3, data))
print('from sort :', list(reversed(sorted(data)[-3:])))
print('3 smallest:', heapq.nsmallest(3, data))
print('from sort :', sorted(data)[:3])
heapq_extremes.py

 运行效果

all       : [19, 9, 4, 10, 11]
3 largest : [19, 11, 10]
from sort : [19, 11, 10]
3 smallest: [4, 9, 10]
from sort : [4, 9, 10]

8、高效合并有序序列

传统的合并方法

list(sorted(itertools.chain(*data)))
# 这个技术可能会占用大量内存。
import heapq
import random

random.seed(2016)

data = []
for i in range(4):
    new_data = list(random.sample(range(1, 101), 5))
    new_data.sort()
    data.append(new_data)

for i, d in enumerate(data):
    print('{}: {}'.format(i, d))

print('\nMerged:')
for i in heapq.merge(*data):
    print(i, end=' ')
print()
heapq_merge.py

运行效果

0: [33, 58, 71, 88, 95]
1: [10, 11, 17, 38, 91]
2: [13, 18, 39, 61, 63]
3: [20, 27, 31, 42, 45]

Merged:
10 11 13 17 18 20 27 31 33 38 39 42 45 58 61 63 71 88 91 95 
posted @ 2020-06-23 10:13  小粉优化大师  阅读(429)  评论(0编辑  收藏  举报