Unique Email Addresses

LeetCode 965 | Difficulty: Easy

Easy

Problem Description

Every valid email consists of a local name and a domain name, separated by the '@' sign. Besides lowercase letters, the email may contain one or more '.' or '+'.

- For example, in `"alice@leetcode.com"`, `"alice"` is the **local name**, and `"leetcode.com"` is the **domain name**.

If you add periods '.' between some characters in the local name part of an email address, mail sent there will be forwarded to the same address without dots in the local name. Note that this rule does not apply to domain names.

- For example, `"alice.z@leetcode.com"` and `"alicez@leetcode.com"` forward to the same email address.

If you add a plus '+' in the local name, everything after the first plus sign will be ignored. This allows certain emails to be filtered. Note that this rule does not apply to domain names.

- For example, `"m.y+name@email.com"` will be forwarded to `"my@email.com"`.

It is possible to use both of these rules at the same time.

Given an array of strings emails where we send one email to each emails[i], return the number of different addresses that actually receive mails.

Example 1:

Input: emails = ["test.email+alex@leetcode.com","test.e.mail+bob.cathy@leetcode.com","testemail+david@lee.tcode.com"]
Output: 2
Explanation: "testemail@leetcode.com" and "testemail@lee.tcode.com" actually receive mails.

Example 2:

Input: emails = ["a@leetcode.com","b@leetcode.com","c@leetcode.com"]
Output: 3

Constraints:

- `1 <= emails.length <= 100`

- `1 <= emails[i].length <= 100`

- `emails[i]` consist of lowercase English letters, `'+'`, `'.'` and `'@'`.

- Each `emails[i]` contains exactly one `'@'` character.

- All local and domain names are non-empty.

- Local names do not start with a `'+'` character.

- Domain names end with the `".com"` suffix.

- Domain names must contain at least one character before `".com"` suffix.

Topics: Array, Hash Table, String

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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.

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Solutions

Solution 1: C# (Best: 104 ms)

Metric	Value
Runtime	104 ms
Memory	28.3 MB
Date	2020-02-21

Solution

public class Solution {
    public int NumUniqueEmails(string[] emails) {
        HashSet<string> uEmails = new HashSet<string>();
    foreach (var email in emails)
    {
        var emailArr  = email.Split('@');
        string emailId = emailArr[0];
        if(emailId.IndexOf('+')!=-1)
        {
            emailId = emailId.Substring(0, emailId.IndexOf('+'));
        }
        uEmails.Add($"{emailId.Replace(".", "")}@{emailArr[1]}");
    }
    return uEmails.Count();
    }
}

📜 1 more C# submission(s)

Submission (2019-02-24) — 116 ms, 26.4 MB

public class Solution {
    public int NumUniqueEmails(string[] emails) {
        HashSet<string> uEmails = new HashSet<string>();
    foreach (var email in emails)
    {
        var emailArr  = email.Split('@');
        string emailId = emailArr[0];
        if(emailId.IndexOf('+')!=-1)
        {
            emailId = emailId.Substring(0, emailId.IndexOf('+'));
        }
        uEmails.Add($"{emailId.Replace(".", "")}@{emailArr[1]}");
    }
    return uEmails.Count();
    }
}

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Complexity Analysis

Approach	Time	Space
Hash Map	$O(n)$	$O(n)$

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Interview Tips

Key Points

• Start by clarifying edge cases: empty input, single element, all duplicates.
• Hash map gives O(1) lookup — think about what to use as key vs value.