Unique Email Addresses

In the local part of an address (before the @), dots are ignored and everything from the first + onward is ignored. The domain is used as-is. How many distinct addresses are there?

numUniqueEmails([]string{
    "test.email+alex@leetcode.com",
    "test.e.mail+bob.cathy@leetcode.com",
    "testemail+david@lee.tcode.com",
})   // 2

Normalise, then count

There is no algorithm here. The whole problem is applying two rules to the right half of the string and putting the results in a set.

func numUniqueEmails(emails []string) int {
    emailSet := make(map[string]struct{})

    for _, email := range emails {
        parts := strings.Split(email, "@")
        local, domain := parts[0], parts[1]

        local = strings.Split(local, "+")[0]
        local = strings.ReplaceAll(local, ".", "")

        emailSet[local+"@"+domain] = struct{}{}
    }

    return len(emailSet)
}

$O(\text{total characters})$, and map[string]struct{} is Go’s idiom for a set.

What Split is doing

strings.Split(email, "@") returns every field, and the code takes parts[1]. That assumes exactly one @, which the constraints guarantee. Without the guarantee, "a@b@c.com" would silently use "b" as the domain and drop the rest.

strings.Cut states the assumption and handles its absence:

local, domain, ok := strings.Cut(email, "@")
if !ok {
    continue   // or return an error — but now the case is visible
}

Cut splits on the first occurrence and reports whether it found one. It also allocates nothing, where Split builds a slice. Since Go 1.18 it is the right default for “split into two at a separator”.

Likewise strings.Split(local, "+")[0] builds every +-separated field to use the first; local, _, _ = strings.Cut(local, "+") builds none.