Grokking the Coding Interview: Patterns for Coding Questions
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Solution: Group Anagrams
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Problem Statement

Given a list of strings, the task is to group the anagrams together.

An anagram is a word or phrase formed by rearranging the letters of another, such as "cinema", formed from "iceman"

You can return the answer in any order.

Examples

Example 1:

  • Input: ["dog", "god", "hello"]
  • Output: [["dog", "god"], ["hello"]]
  • Justification: "dog" and "god" are anagrams, so they are grouped together. "hello" does not have any anagrams in the list, so it is in its own group.

Example 2:

  • Input: ["listen", "silent", "enlist"]
  • Output: [["listen", "silent", "enlist"]]
  • Justification: All three words are anagrams of each other, so they are grouped together.

Example 3:

  • Input: ["abc", "cab", "bca", "xyz", "zxy"]
  • Output: [["abc", "cab", "bca"], ["xyz", "zxy"]]
  • Justification: "abc", "cab", and "bca" are anagrams, as are "xyz" and "zxy".

Constraints:

  • 1 <= strs.length <= 10<sup>4</sup>
  • 0 <= strs[i].length <= 100
  • strs[i] consists of lowercase English letters.

Solution

  • Sorting Approach:

    • For each word in the input list:
      • Sort the letters of the word.
      • Use the sorted word as a key in a hash map, and add the original word to the list of values for that key.
    • The hash map values will be the groups of anagrams.

  • Why This Will Work:

    • Anagrams will always result in the same sorted word, so they will be grouped together in the hash map.

Algorithm Walkthrough

  • Given the input ["abc", "cab", "bca", "xyz", "zxy"]
  • For "abc":
    • Sorted word is "abc".
    • Add "abc" to the hash map with key "abc".
  • For "cab":
    • Sorted word is "abc".
    • Add "cab" to the list in the hash map with key "abc".
  • Continue this process for all words.
  • The hash map values are the groups of anagrams.

Code

Python3
Python3
. . . .

Complexity Analysis

  • Time Complexity: O(n*k*log(k)), where n is the number of strings, and k is the maximum length of a string in strs. This is because each of the n strings is sorted in O(k*log(k)) time.
  • Space Complexity: O(n*k), where n is the number of strings, and k is the maximum length of a string in strs. This space is used for the output data structure and the hash map.

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