python list and dict skills

Author: PegasusWang

docs/19_python内置常用算法和数据结构/builtins.md

@@ -98,19 +98,9 @@ class Solution:
 - https://www.runoob.com/w3cnote/python-negative-storage.html
 - https://leetcode-cn.com/problems/convert-integer-lcci/solution/python3-zhu-yi-qi-dui-yu-fu-shu-de-cun-chu-fang-sh/
 
-# python list/dict 排序等技巧
+# python list 技巧
 
 ```py
-# python 根据 key，value 排序字典
-d = {'d': 4, 'a': 1, 'b': 2, 'c':3}
-# dict sort by key and reverse
-dict(sorted(d.items()))  # {'a': 1, 'b': 2, 'c': 3, 'd': 4}
-dict(sorted(d.items(), reverse=True)) # {'d': 4, 'c': 3, 'b': 2, 'a': 1}
-
-# dict sort by value and reverse
-dict(sorted(d.items(), key = lambda kv:kv[1])) # {'a': 1, 'b': 2, 'c': 3, 'd': 4}
-dict(sorted(d.items(), key = lambda kv:kv[1], reverse=True)) # {'d': 4, 'c': 3, 'b': 2, 'a': 1}
-
 # 排序嵌套 list
 l = [('a', 1), ('c', 2), ('b',3)]
 sorted(l, key=lambda p:p[0]) # 根据第1个值排序，[('a', 1), ('b', 3), ('c', 2)]
@@ -120,18 +110,11 @@ sorted(l, key=lambda p:p[1]) # 根据第2个值排序，[('a', 1), ('c', 2), ('b
 l = [1,2,5,4,3]
 maxi, maxval = max(enumerate(l), key=lambda iv: iv[1]) # 2, 5
 
-# 获取字典对应的最大值对应的 key,value
-mydict = {'A':4,'B':10,'C':0,'D':87}
-maximum = max(mydict, key=mydict.get)  # Just use 'min' instead of 'max' for minimum.
-maxk, maxv = maximum, mydict[maximum]
-# 或者
-maxk, maxv = max(mydict.items(), key=lambda k: k[1])
-
-# python3 排序list自定义函数(python2 直接用 cmp 参数)
+# python3 排序list自定义函数(python2 直接用 cmp 参数， python3 需要用 cmp_to_key 转成 key 参数)
 from functools import cmp_to_key
 nums = [3,2,1,4,5]
-sorted(nums, key= cmp_to_key(lambda a,b: a-b) ) # [1 ,2 ,3, 4, 5]
-sorted(nums, key= cmp_to_key(lambda a,b: b-a) ) # [5, 4, 3, 2, 1]
+sorted(nums, key=cmp_to_key(lambda a,b: a-b) ) # [1 ,2 ,3, 4, 5]
+sorted(nums, key=cmp_to_key(lambda a,b: b-a) ) # [5, 4, 3, 2, 1]
 
 # 一行代码判断列表是否有序
 issorted = all(l[i] <= l[i+1] for i in range(len(l) - 1))
@@ -140,10 +123,36 @@ issorted = all(l[i] <= l[i+1] for i in range(len(l) - 1))
 from itertools import accumulate
 presums = list(accumulate([1,2,3])) # [1, 3, 6]
 
-# 一行代码求矩阵和 https://stackoverflow.com/questions/10713150/how-to-sum-a-2d-array-in-python
+# 一行代码求矩阵元素总和 https://stackoverflow.com/questions/10713150/how-to-sum-a-2d-array-in-python
 allsum = sum(map(sum, matrix)) # 或者 allsum = sum((sum(row) for row in matrix))
 ```
 
+# python dict 技巧
+
+```py
+# python 根据 key，value 排序字典
+d = {'d': 4, 'a': 1, 'b': 2, 'c':3}
+# dict sort by **key** and reverse
+dict(sorted(d.items()))  # {'a': 1, 'b': 2, 'c': 3, 'd': 4}
+dict(sorted(d.items(), reverse=True)) # {'d': 4, 'c': 3, 'b': 2, 'a': 1}
+
+# dict sort by **value** and reverse
+dict(sorted(d.items(), key = lambda kv:kv[1])) # {'a': 1, 'b': 2, 'c': 3, 'd': 4}
+dict(sorted(d.items(), key = lambda kv:kv[1], reverse=True)) # {'d': 4, 'c': 3, 'b': 2, 'a': 1}
+
+# 获取字典对应的最大值对应的 key,value
+mydict = {'A':4,'B':10,'C':0,'D':87}
+maximum = max(mydict, key=mydict.get)  # Just use 'min' instead of 'max' for minimum.
+maxk, maxv = maximum, mydict[maximum]
+# 或者
+maxk, maxv = max(mydict.items(), key=lambda k: k[1])
+
+# 支持默认值的有序字典 (OrderedDict and defaultdict) 
+# https://stackoverflow.com/questions/6190331/how-to-implement-an-ordered-default-dict
+od = OrderedDict()  # collections.OrderedDict()
+od[i] = od.get(i, 0) + 1 # 间接实现了 defaultdict(int) ，同时保持了插入字典的 key 顺序
+```
+
 # 链表题目调试函数
 
 ```py