字符串操作、正则（零宽断言）：2种方法求解《929. 独特的电子邮件地址》

var numUniqueEmails = function(emails) { // split 实现
  const n = emails.length, h = new Set()
  for (let i = 0; i < n; i++) {
    const [local, domain] = emails[i].split('@')
    h.add(local.split('+')[0].replace(/\./g, '') + '@' + domain)
  }
  return h.size
};

var numUniqueEmails = function(emails) { // slice 或 substring 或 substr 实现
  const n = emails.length, h = new Set()
  for (let i = 0; i < n; i++) {
    const j = emails[i].indexOf('@'), l = emails[i].slice(0, j), r = emails[i].slice(j), k = l.indexOf('+')
    h.add((k > - 1 ? l.slice(0, k) : l).replaceAll('.', '') + r)
  }
  return h.size
};

function numUniqueEmails(emails: string[]): number {
  const h = new Set()
  for (const email of emails) {
    const i = email.indexOf('@'), l = email.substring(0, i), r = email.substring(i), j = l.indexOf('+')
    h.add((j > -1 ? l.substring(0, j) : l).replace(/\./g, '') + r)
  }
  return h.size
};

class Solution {
  function numUniqueEmails($emails) {
    $h = array();
    foreach($emails as $email) {
      $i = strpos($email, '@');
      $l = substr($email, 0, $i);
      $r = substr($email, $i);
      $j = strpos($l, '+');
      $h[str_replace('.', '', $j > -1 ? substr($l, 0, $j) : $l) . $r] = 1;
    }
    return count(array_keys($h));
  }
}

func numUniqueEmails(emails []string) int { // Split 实现
  type void struct{}
  var value void
  h := map[string]void{}
  for _, email := range emails {
    s := strings.SplitN(email, "@", 2)
    l, r := s[0], s[1]
    s = strings.SplitN(l, "+", 2)
    l = s[0]
    l = strings.ReplaceAll(l, ".", "")
    h[l + "@" + r] = value
  }
  return len(h)
}

func numUniqueEmails(emails []string) int { // Index 实现
  type void struct{}
  var value void 
  h := map[string]void{}
  for _, email := range emails {
    i := strings.IndexByte(email, '@')
    l, r := email[:i], email[i:]
    j := strings.IndexByte(l, '+')
    if j > -1 {
      l = email[:j]
    }
    l = strings.ReplaceAll(l, ".", "")
    h[l + r] = value
  }
  return len(h)
}

class Solution { // split 实现
  public int numUniqueEmails(String[] emails) {
    int n = emails.length;
    HashSet<String> h = new HashSet<String>();
    for (int i = 0; i < n; i++) {
      String[] a = emails[i].split("@", 2);
      String l = a[0];
      String r = a[1];
      a = l.split("\\+", 2);
      l = a[0];
      h.add(l.replaceAll("\\.", "") + "@" + r);
    }
    return h.size();
  }
}

class Solution { // indexOf 实现
  public int numUniqueEmails(String[] emails) {
    int n = emails.length;
    HashSet<String> h = new HashSet<String>();
    for (int i = 0; i < n; i++) {
      int j = emails[i].indexOf("@");
      String l = emails[i].substring(0, j);
      String r = emails[i].substring(j);
      int k = l.indexOf("+");
      if (k > -1) l = l.substring(0, k);
      h.add(l.replaceAll("\\.", "") + r);
    }
    return h.size();
  }
}

class Solution: # split 实现
  def numUniqueEmails(self, emails: List[str]) -> int:
    h = set()
    for _, email in enumerate(emails):
      l1 = email.split('@', 2)
      l, r = l1[0], l1[1]
      l2 = l.split('+', 2)
      l = l2[0]
      l = l.replace('.', '')
      h.add(l + '@' + r)
    return len(h)

class Solution: # find 实现
  def numUniqueEmails(self, emails: List[str]) -> int:
    h = set()
    for _, email in enumerate(emails):
      i = email.find("@")
      l, r = email[:i], email[i:]
      j = l.find("+")
      if j > -1 : l = l[:j]
      l = l.replace('.', '')
      h.add(l + r)
    return len(h)

var numUniqueEmails = function(emails) {
  const n = emails.length, h = new Set()
  for (let i = 0; i < n; i++) {
    h.add(emails[i].replace(/(\.|\+.*)(?=.*@)/g, ''))
  }
  return h.size
};

var numUniqueEmails = function(emails) {
  const n = emails.length, h = new Set()
  for (let i = 0; i < n; i++) {
    h.add(emails[i].replace(/\.(?=.*@)|\+[^@]*/g, ''))
  }
  return h.size
};

function numUniqueEmails(emails: string[]): number {
  const h = new Set()
  for (const email of emails) {
    h.add(email.replace(/(\.|\+.*)(?=.*@)/g, ''))
  }
  return h.size
};

class Solution {
  function numUniqueEmails($emails) {
    $h = array();
    foreach($emails as $email) {
      $h[preg_replace('/(\.|\+.*)(?=.*@)/', '', $email)] = 1;
    }
    return count(array_keys($h));
  }
}

class Solution {
  public int numUniqueEmails(String[] emails) {
    int n = emails.length;
    HashSet<String> h = new HashSet<String>();
    for (int i = 0; i < n; i++) {
      h.add(emails[i].replaceAll("(\\.|\\+.*)(?=.*@)", ""));
    }
    return h.size();
  }
}

class Solution:
  def numUniqueEmails(self, emails: List[str]) -> int:
    h = set()
    for _, email in enumerate(emails):
      h.add(re.sub(r'(\.|\+.*)(?=.*@)', '', email))
    return len(h)