929. Unique Email Addresses.md

929. Unique Email Addresses

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

For example, in [email protected], 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, "[email protected]" and "[email protected]" 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 [email protected] will be forwarded to [email protected]. (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?

Example 1:

Input: ["[email protected]","[email protected]","[email protected]"]
Output: 2
Explanation: "[email protected]" and "[email protected]" actually receive mails

Solution

• mine

// O(n)time O(n)space
class Solution {
    public int numUniqueEmails(String[] emails) {
        Set<String> res = new HashSet<>();
        for(String email : emails){
            res.add(getShortEmail(email));
        }
        return res.size();
    }
    
    public String getShortEmail(String email){
        String temp = email.substring(0, email.indexOf("@"));
        String end = email.substring(email.indexOf("@"), email.length());
        temp = temp.replaceAll("\\.", "");
        int plus = temp.indexOf("+");
        if(plus == -1){
            return temp + end;
        }else{
            return temp.substring(0,plus) + end;
        }
    }
    
}

• the most votes

public int numUniqueEmails(String[] emails) {
    Set<String> normalized = new HashSet<>(); // used to save simplified email address, cost O(n) sapce.
    for (String email : emails) {
        String[] parts = email.split("@"); // split into local and domain parts.
        String[] local = parts[0].split("\\+"); // split local by '+'.
        normalized.add(local[0].replace(".", "") + "@" + parts[1]); // remove all '.', and concatenate '@' and domain.        
    }
    return normalized.size();
}