Group Anagrams

LeetCode 49 | Difficulty: Medium

Medium

Problem Description

Given an array of strings strs, group the anagrams together. You can return the answer in any order.

Example 1:

Input: strs = ["eat","tea","tan","ate","nat","bat"]

Output: [["bat"],["nat","tan"],["ate","eat","tea"]]

Explanation:

- There is no string in strs that can be rearranged to form `"bat"`.

- The strings `"nat"` and `"tan"` are anagrams as they can be rearranged to form each other.

- The strings `"ate"`, `"eat"`, and `"tea"` are anagrams as they can be rearranged to form each other.

Example 2:

Input: strs = [""]

Output: [[""]]

Example 3:

Input: strs = ["a"]

Output: [["a"]]

Constraints:

- `1 <= strs.length <= 10^4`

- `0 <= strs[i].length <= 100`

- `strs[i]` consists of lowercase English letters.

Topics: Array, Hash Table, String, Sorting

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Approach

Hash Map

Use a hash map for O(1) average lookups. Store seen values, frequencies, or indices. The key question: what should I store as key, and what as value?

When to use

Need fast lookups, counting frequencies, finding complements/pairs.

String Processing

Consider character frequency counts, two-pointer approaches, or building strings efficiently. For pattern matching, think about KMP or rolling hash. For palindromes, expand from center or use DP.

When to use

Anagram detection, palindrome checking, string transformation, pattern matching.

Sorting

Sort the input to bring related elements together or enable binary search. Consider: does sorting preserve the answer? What property does sorting give us?

When to use

Grouping, finding closest pairs, interval problems, enabling two-pointer or binary search.

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Solutions

Solution 1: C# (Best: 452 ms)

Metric	Value
Runtime	452 ms
Memory	39.1 MB
Date	2020-04-06

Solution

public class Solution {
    public IList<IList<string>> GroupAnagrams(string[] strs) {
            Dictionary<string, List<string>> ana = new Dictionary<string, List<string>>();
    foreach (var anagram in strs)
    {
        var anagramArray = anagram.ToCharArray();
        Array.Sort(anagramArray);
        var sorted = new String(anagramArray);
        if(ana.ContainsKey(sorted))
        {
            ana[sorted].Add(anagram);
        }
        else 
        {           
            ana.Add(sorted, new List<string>{anagram});
        }
    }
    IList<IList<string>> anagrams =  ana.Select(x=>(IList<string>)(x.Value)).ToList();
    
    return anagrams;
    }
}

📜 1 more C# submission(s)

Submission (2018-01-16) — 624 ms, N/A

public class Solution {
    public IList<IList<string>> GroupAnagrams(string[] strs) {
            Dictionary<string, List<string>> ana = new Dictionary<string, List<string>>();
    foreach (var anagram in strs)
    {
        var anagramArray = anagram.ToCharArray();
        Array.Sort(anagramArray);
        var sorted = new String(anagramArray);
        if(ana.ContainsKey(sorted))
        {
            ana[sorted].Add(anagram);
        }
        else 
        {           
            ana.Add(sorted, new List<string>{anagram});
        }
    }
    IList<IList<string>> anagrams =  ana.Select(x=>(IList<string>)(x.Value)).ToList();
    
    return anagrams;
    }
}

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Complexity Analysis

Approach	Time	Space
Sort + Process	$O(n log n)$	$O(1) to O(n)$
Hash Map	$O(n)$	$O(n)$

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Interview Tips

Key Points

• Discuss the brute force approach first, then optimize. Explain your thought process.
• Hash map gives O(1) lookup — think about what to use as key vs value.