Learning python by leetcode: No.49 Group Anagrams

题目

leetcode 49 Group Anagrams

code

class Solution:
    def groupAnagrams(self, strs):
        """
        :type strs: List[str]
        :rtype: List[List[str]]
        """
        result = collections.defaultdict(list)
        for word in strs:
            count = [0] *26
            for c in word:
                count[ord(c) - ord('a')] +=1
            result[tuple(count)].append(word)
        return list(result.values())

FBI Warning

In python2 , dict.values() return a copy of the dict list;
However, in python3, dict.values() return view objects. They provide a dynamic view on the dictionary’s entries, which means that when the dictionary changes, the view reflects these changes.

As a result, you need to encapsulate it into list by list().

details

line 7

defaultdict is a subclass of dict;
defaultdict has default value for the keys (for dict, you the key-value doesn’t exist, throw Keyerror!). You specify the default value by giving a default_factory( it defaults to None. )
The factory may be function or lambda.

line 12

The key of a dict must be invariable object.
List is a variable object, unsuitable for key, OK with tuple.

不可变对象，该对象所指向的内存中的值不能被改变。当改变某个变量时候，由于其所指的值不能被改变，相当于把原来的值复制一份后再改变，这会开辟一个新的地址，变量再指向这个新的地址。
可变对象，该对象所指向的内存中的值可以被改变。变量（准确的说是引用）改变后，实际上是其所指的值直接发生改变，并没有发生复制行为，也没有开辟新的出地址，通俗点说就是原地改变。
Python中，数值类型（int和float）、字符串str、元组tuple都是不可变类型。而列表list、字典dict、集合set是可变类型。