LeetCode #929 — EASY

Unique Email Addresses

Build confidence with an intuition-first walkthrough focused on array fundamentals.

The Problem

Problem Statement

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.

Step 01

Brute Force Baseline

Problem summary: 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

Baseline thinking

Start with the most direct exhaustive search. That gives a correctness anchor before optimizing.

Pattern signal: Array · Hash Map

Example 1

["test.email+alex@leetcode.com","test.e.mail+bob.cathy@leetcode.com","testemail+david@lee.tcode.com"]

Example 2

["a@leetcode.com","b@leetcode.com","c@leetcode.com"]

Step 02

Core Insight

What unlocks the optimal approach

No official hints in dataset. Start from constraints and look for a monotonic or reusable state.

Interview move: turn each hint into an invariant you can check after every iteration/recursion step.

Step 03

Algorithm Walkthrough

Iteration Checklist

Define state (indices, window, stack, map, DP cell, or recursion frame).
Apply one transition step and update the invariant.
Record answer candidate when condition is met.
Continue until all input is consumed.

Use the first example testcase as your mental trace to verify each transition.

Step 04

Edge Cases

Minimum Input

Single element / shortest valid input

Validate boundary behavior before entering the main loop or recursion.

Duplicates & Repeats

Repeated values / repeated states

Decide whether duplicates should be merged, skipped, or counted explicitly.

Extreme Constraints

Upper-end input sizes

Re-check complexity target against constraints to avoid time-limit issues.

Invalid / Corner Shape

Empty collections, zeros, or disconnected structures

Handle special-case structure before the core algorithm path.

Step 05

Full Annotated Code

Source-backed implementations are provided below for direct study and interview prep.

// Accepted solution for LeetCode #929: Unique Email Addresses
class Solution {
    public int numUniqueEmails(String[] emails) {
        Set<String> s = new HashSet<>();
        for (String email : emails) {
            String[] parts = email.split("@");
            String local = parts[0];
            String domain = parts[1];
            StringBuilder t = new StringBuilder();
            for (char c : local.toCharArray()) {
                if (c == '.') {
                    continue;
                }
                if (c == '+') {
                    break;
                }
                t.append(c);
            }
            s.add(t.toString() + "@" + domain);
        }
        return s.size();
    }
}

// Accepted solution for LeetCode #929: Unique Email Addresses
func numUniqueEmails(emails []string) int {
	s := make(map[string]struct{})
	for _, email := range emails {
		parts := strings.Split(email, "@")
		local := parts[0]
		domain := parts[1]
		var t strings.Builder
		for _, c := range local {
			if c == '.' {
				continue
			}
			if c == '+' {
				break
			}
			t.WriteByte(byte(c))
		}
		s[t.String()+"@"+domain] = struct{}{}
	}
	return len(s)
}

# Accepted solution for LeetCode #929: Unique Email Addresses
class Solution:
    def numUniqueEmails(self, emails: List[str]) -> int:
        s = set()
        for email in emails:
            local, domain = email.split("@")
            t = []
            for c in local:
                if c == ".":
                    continue
                if c == "+":
                    break
                t.append(c)
            s.add("".join(t) + "@" + domain)
        return len(s)

// Accepted solution for LeetCode #929: Unique Email Addresses
use std::collections::HashSet;

impl Solution {
    pub fn num_unique_emails(emails: Vec<String>) -> i32 {
        let mut s = HashSet::new();

        for email in emails {
            let parts: Vec<&str> = email.split('@').collect();
            let local = parts[0];
            let domain = parts[1];
            let mut t = String::new();
            for c in local.chars() {
                if c == '.' {
                    continue;
                }
                if c == '+' {
                    break;
                }
                t.push(c);
            }
            s.insert(format!("{}@{}", t, domain));
        }

        s.len() as i32
    }
}

// Accepted solution for LeetCode #929: Unique Email Addresses
function numUniqueEmails(emails: string[]): number {
    const s = new Set<string>();
    for (const email of emails) {
        const [local, domain] = email.split('@');
        let t = '';
        for (const c of local) {
            if (c === '.') {
                continue;
            }
            if (c === '+') {
                break;
            }
            t += c;
        }
        s.add(t + '@' + domain);
    }
    return s.size;
}

Step 06

Interactive Study Demo

Use this to step through a reusable interview workflow for this problem.

Custom checkpoints (one per line)

Press Step or Run All to begin.

Step 07

Complexity Analysis

Time

O(L)

Space

O(L)

Approach Breakdown

BRUTE FORCE

O(n²) time

O(1) space

Two nested loops check every pair or subarray. The outer loop fixes a starting point, the inner loop extends or searches. For n elements this gives up to n²/2 operations. No extra space, but the quadratic time is prohibitive for large inputs.

OPTIMIZED

O(n) time

O(1) space

Most array problems have an O(n²) brute force (nested loops) and an O(n) optimal (single pass with clever state tracking). The key is identifying what information to maintain as you scan: a running max, a prefix sum, a hash map of seen values, or two pointers.

Shortcut: If you are using nested loops on an array, there is almost always an O(n) solution. Look for the right auxiliary state.

Coach Notes

Common Mistakes

Review these before coding to avoid predictable interview regressions.

Off-by-one on range boundaries

Wrong move: Loop endpoints miss first/last candidate.

Usually fails on: Fails on minimal arrays and exact-boundary answers.

Fix: Re-derive loops from inclusive/exclusive ranges before coding.

Mutating counts without cleanup

Wrong move: Zero-count keys stay in map and break distinct/count constraints.

Usually fails on: Window/map size checks are consistently off by one.

Fix: Delete keys when count reaches zero.