# 929. Unique Email Addresses

Every email consists of a local name and a domain name, separated by the @ sign.

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

Besides lowercase letters, these emails may contain `'.'`s or `'+'`s.

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.  For example, `"alice.z@leetcode.com"` and `"alicez@leetcode.com"` forward to the same email address.  (Note that this rule does not apply for domain names.)

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, for example `m.y+name@email.com` will be forwarded to `my@email.com`.  (Again, this rule does not apply for domain names.)

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

Given a list of `emails`, we send one email to each address in the list.  How many different addresses actually receive mails?&#x20;

**Example 1:**

```
Input: ["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
```

**Note:**

* `1 <= emails[i].length <= 100`
* `1 <= emails.length <= 100`
* Each `emails[i]` contains exactly one `'@'` character.

{% hint style="info" %}
遇到+號就忽略後方的字串，遇到.號就跳過該字元繼續往下掃描形成一個子字串然後存到一個hashset裡。
{% endhint %}

```cpp
int numUniqueEmails(vector<string>& emails) { // time: O(n * str_len); space: O(n)
    unordered_set<string> valid_emails;
    for (string& email : emails) {
        int at_pos = email.find('@');
        string local;
        for (int i = 0; i < at_pos; ++i) {
            if (email[i] == '+') break;
            if (email[i] == '.') continue;
            local += email[i];
        }
        string valid_email = local + email.substr(at_pos);
        valid_emails.insert(valid_email);
    }
    return valid_emails.size();
}
```
