Group Anagrams

Identifying Problem Isomorphism

“Group Anagrams” involves sorting strings to form keys in a hash map, and then using these keys to group anagrams together.

A simpler problem that is isomorphic is “Two Strings are Anagrams”. In this problem, you sort two strings and compare them to determine if they are anagrams. This problem is simpler because it only requires comparing two strings rather than grouping multiple strings.

A more complex problem that has an approximate isomorphism is “Find All Anagrams in a String”. In this problem, you use a sliding window to find all anagrams of a string p in a string s. This is more complex because it involves moving a window across the string s and comparing substrings with the string p.

In increasing order of complexity, the problems are:

  1. “Two Strings are Anagrams” - Compare two sorted strings to determine if they are anagrams.
  2. “Group Anagrams” - Sort strings to form keys and use these keys to group anagrams together.
  3. “Find All Anagrams in a String” - Use a sliding window to find all anagrams of a string p in a string s.

10 Prerequisite LeetCode Problems

The problem “Group Anagrams” involves string manipulation and hashing. Here are 10 problems to prepare:

  1. Valid Anagram (LeetCode 242): This problem is a simpler version of “Group Anagrams”. It only involves checking if two given strings are anagrams.

  2. First Unique Character in a String (LeetCode 387): This problem can help you understand the basic manipulation of strings and characters in a string.

  3. Longest Palindromic Substring (LeetCode 5): This problem can help you improve your string manipulation skills.

  4. Count and Say (LeetCode 38): This problem also involves string manipulation. It can help you improve your understanding of how to work with strings.

  5. Two Sum (LeetCode 1): This problem involves using a hash map to find a pair of elements that satisfy certain conditions.

  6. Contains Duplicate (LeetCode 217): This problem also involves using a hash map. It’s a simpler problem that can help you understand how to use hash maps.

  7. Intersection of Two Arrays II (LeetCode 350): This problem involves using a hash map to find the intersection of two arrays. It’s a good practice problem for learning how to use hash maps.

  8. Jewels and Stones (LeetCode 771): This problem involves using a hash set to find the count of specific characters in a string.

  9. Single Number (LeetCode 136): This problem requires you to use hash maps and bit manipulation.

  10. Isomorphic Strings (LeetCode 205): This problem requires understanding of string manipulation and the usage of hash maps. It also involves a concept similar to group anagrams, where you need to match patterns between two strings.

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13

class Solution:
    def groupAnagrams(self, strs: List[str]) -> List[List[str]]:
        strs_table = {}

        for string in strs:
            sorted_string = ''.join(sorted(string))

            if sorted_string not in strs_table:
                strs_table[sorted_string] = []

            strs_table[sorted_string].append(string)

        return list(strs_table.values())

You’re looking to group anagrams from a list of strings. Here’s a Python3 code snippet that implements the solution:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11

from collections import defaultdict

class Solution:
    def groupAnagrams(self, strs: List[str]) -> List[List[str]]:
        anagram_groups = defaultdict(list)

        for s in strs:
            sorted_str = ''.join(sorted(s))
            anagram_groups[sorted_str].append(s)

        return list(anagram_groups.values())

Explanation:

  1. We import defaultdict from the collections module to create a dictionary where the default values are lists.
  2. In the groupAnagrams method, we initialize a defaultdict named anagram_groups.
  3. We iterate through the given strings, sorting each string, and use the sorted string as a key in the dictionary. Anagrams will result in the same sorted string, so they will be grouped under the same key.
  4. We append the original (unsorted) string to the list corresponding to its sorted key.
  5. Finally, we return the lists from the dictionary, which contain the grouped anagrams.

This approach leverages the fact that anagrams, when sorted, yield the same string. By sorting each string and using the sorted version as a key in a dictionary, we can easily group the anagrams together.

Language Agnostic Coding Drills

  1. Understanding Data Types: Understand the basics of data types including strings, lists, and dictionaries.

  2. Defining a Class and a Method: Understanding the structure of a class and how to define methods within a class.

  3. Looping over a List: Be able to use a for-loop to iterate over a list.

  4. Working with Strings: Understand how to sort a string and how to join a list of characters back into a string.

  5. Working with Dictionaries: Learn how to use a dictionary to group items together, access dictionary values, check if a key exists in a dictionary, and append items to a list in a dictionary.

  6. Working with Lists: Learn how to add items to a list and convert dictionary values to a list.

  7. Returning Values from a Function: Understand how to use the return statement to produce an output from a function.

Arranging these concepts in increasing level of difficulty:

  1. Understanding Data Types
  2. Defining a Class and a Method
  3. Looping over a List
  4. Working with Strings
  5. Working with Lists
  6. Working with Dictionaries
  7. Returning Values from a Function

Targeted Drills in Python

  1. Understanding Data Types: Write a piece of code that declares a string, a list, and a dictionary. Print the type of each variable.

    1

    # Your code here

  2. Defining a Class and a Method: Create a class Animal with a method speak that prints “Unknown animal”.

    1

    # Your code here

  3. Looping over a List: Given a list of integers, iterate over the list and print each integer.

    1
2

    numbers = [1, 2, 3, 4, 5]
# Your code here

  4. Working with Strings: Given a string, write a function that sorts the characters in the string and returns the sorted string.

    1
2
3
4

    def sort_string(s):
    # Your code here

print(sort_string("hello"))  # should print "ehllo"

  5. Working with Lists: Write a function that takes a list as input and appends an element to the list.

    1
2
3
4
5
6

    def append_to_list(lst, elem):
    # Your code here

numbers = [1, 2, 3]
append_to_list(numbers, 4)
print(numbers)  # should print "[1, 2, 3, 4]"

  6. Working with Dictionaries: Write a function that takes a dictionary, a key and a value. The function should add the key-value pair to the dictionary and print the dictionary.

    1
2
3
4
5
6

    def add_to_dict(d, key, value):
    # Your code here

my_dict = {"one": 1, "two": 2}
add_to_dict(my_dict, "three", 3)
# should print "{'one': 1, 'two': 2, 'three': 3}"

  7. Returning Values from a Function: Write a function add_numbers that takes two integers and returns their sum.

    1
2
3
4

    def add_numbers(a, b):
    # Your code here

print(add_numbers(3, 4))  # should print "7"