929. Unique Email Addresses - Explanation

1. Built-In Functions

class Solution:
    def numUniqueEmails(self, emails: List[str]) -> int:
        unique = set()

        for e in emails:
            local, domain = e.split('@')
            local = local.split("+")[0]
            local = local.replace(".", "")
            unique.add((local, domain))
        return len(unique)

Time & Space Complexity

Time complexity: $O(n * m)$
Space complexity: $O(n)$

Where $n$ is the number of strings in the array, and $m$ is the average length of these strings.

2. Iteration

class Solution:
    def numUniqueEmails(self, emails: List[str]) -> int:
        unique = set()

        for e in emails:
            i, local = 0, ""
            while e[i] not in ["@", "+"]:
                if e[i] != ".":
                    local += e[i]
                i += 1

            while e[i] != "@":
                i += 1
            domain = e[i + 1:]
            unique.add((local, domain))
        return len(unique)

Time & Space Complexity

Time complexity: $O(n * m)$
Space complexity: $O(n)$

Where $n$ is the number of strings in the array, and $m$ is the average length of these strings.