Day 5: Group Anagrams - leetcode - Python3

Given an array of strings strs, group the anagrams together. You can return the answer in any order.
An Anagram is a word or phrase formed by rearranging the letters of a different word or phrase, typically using all the original letters exactly once.
 
Example 1:
Input: strs = ["eat","tea","tan","ate","nat","bat"]
Output: [["bat"],["nat","tan"],["ate","eat","tea"]]
Example 2:
Input: strs = [""]
Output: [[""]]
Example 3:
Input: strs = ["a"]
Output: [["a"]]
 
Constraints:
1 <= strs.length <= 104
0 <= strs[i].length <= 100
strs[i] consists of lowercase English letters.

SOLUTION:

class Solution:
    def groupAnagrams(self, strs: List[str]) -> List[List[str]]:
        res: defaultdict[list]=defaultdict(list)
        for word in strs:
            count=[0] * 26 #a-z
            for char in word:
                count[ord(char) - ord("a")] +=1 #KEY
            res[tuple(count)].append(word)
            #converts the list to a tuple, and uses it as
            #a key to store the words in the res dictionary
        return res.values()
Time complexity:
$O(n+m)$
Space complexity:
O(n+m)O(n + m)O(n + u), where n is the number of words in strs and u is the number of unique anagram patterns.
Let's break down how the code works:
The code defines a class named Solution that contains a method groupAnagrams. The method takes a parameter strs, which is a list of strings, and returns a list of lists of strings.
It initializes a defaultdict named res using the collections module. This dictionary-like object will automatically create an empty list as the default value for any key that does not exist.
It iterates over each word in the input list strs.
For each word, it creates a count list of 26 integers initialized to 0. This list will be used to count the frequency of each character in the word.
It iterates over each char in the current word.
For each char, it calculates the index by subtracting the ASCII value of "a" from the ASCII value of the current character using the ord() function. This index represents the corresponding position in the count list.
It increments the count at the calculated index by 1.
It converts the count list to a tuple and uses it as a key to access the res dictionary.
It appends the current word to the list associated with the calculated key in the res dictionary.
After iterating over all the words, it returns the values of the res dictionary as the final result. The values are a list of lists, where each inner list contains anagrams grouped together.
 In summary, It counts the frequency of each character in each word using a count list, converts the list to a tuple, and uses it as a key to store the words in the res dictionary. Finally, it returns the grouped anagrams as a list of lists.