description: n people gathered to hold a jury meeting of the upcoming competition, the i-th member of the jury came up with a_i tasks, which they want to share with each other.
  
  First, the jury decides on the order which they will follow while describing the tasks. Let that be a permutation p of numbers from 1 to n (an array of size n where each integer from 1 to n occurs exactly once).
  
  Then the discussion goes as follows:
  
    * If a jury member p_1 has some tasks left to tell, then they tell one task to others. Otherwise, they are skipped. 
    * If a jury member p_2 has some tasks left to tell, then they tell one task to others. Otherwise, they are skipped. 
    * ... 
    * If a jury member p_n has some tasks left to tell, then they tell one task to others. Otherwise, they are skipped. 
    * If there are still members with tasks left, then the process repeats from the start. Otherwise, the discussion ends. 
  
  
  
  A permutation p is nice if none of the jury members tell two or more of their own tasks in a row. 
  
  Count the number of nice permutations. The answer may be really large, so print it modulo 998 244 353.
  
  Input
  
  The first line contains a single integer t (1 ≤ t ≤ 10^4) — the number of test cases.
  
  The first line of the test case contains a single integer n (2 ≤ n ≤ 2 ⋅ 10^5) — number of jury members.
  
  The second line contains n integers a_1, a_2, ..., a_n (1 ≤ a_i ≤ 10^9) — the number of problems that the i-th member of the jury came up with.
  
  The sum of n over all test cases does not exceed 2 ⋅ 10^5.
  
  Output
  
  For each test case, print one integer — the number of nice permutations, taken modulo 998 244 353.
  
  Example
  
  Input
  
  
  4
  2
  1 2
  3
  5 5 5
  4
  1 3 3 7
  6
  3 4 2 1 3 3
  
  
  Output
  
  
  1
  6
  0
  540
  
  Note
  
  Explanation of the first test case from the example:
  
  There are two possible permutations, p = [1, 2] and p = [2, 1]. For p = [1, 2], the process is the following:
  
    1. the first jury member tells a task; 
    2. the second jury member tells a task; 
    3. the first jury member doesn't have any tasks left to tell, so they are skipped; 
    4. the second jury member tells a task. 
  
  
  
  So, the second jury member has told two tasks in a row (in succession), so the permutation is not nice.
  
  For p = [2, 1], the process is the following:
  
    1. the second jury member tells a task; 
    2. the first jury member tells a task; 
    3. the second jury member tells a task. 
  
  
  
  So, this permutation is nice.
solution: import os
  import sys
  from io import BytesIO, IOBase
  
  _print = print
  BUFSIZE = 8192
  
  
  def dbg(*args, **kwargs):
      _print('\33[95m', end='')
      _print(*args, **kwargs)
      _print('\33[0m', end='')
  
  
  class FastIO(IOBase):
      newlines = 0
  
      def __init__(self, file):
          self._fd = file.fileno()
          self.buffer = BytesIO()
          self.writable = 'x' in file.mode or 'r' not in file.mode
          self.write = self.buffer.write if self.writable else None
  
      def read(self):
          while True:
              b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
              if not b:
                  break
              ptr = self.buffer.tell()
              self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
          self.newlines = 0
          return self.buffer.read()
  
      def readline(self):
          while self.newlines == 0:
              b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
              self.newlines = b.count(b'\n') + (not b)
              ptr = self.buffer.tell()
              self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
          self.newlines -= 1
          return self.buffer.readline()
  
      def flush(self):
          if self.writable:
              os.write(self._fd, self.buffer.getvalue())
              self.buffer.truncate(0), self.buffer.seek(0)
  
  
  class IOWrapper(IOBase):
      def __init__(self, file):
          self.buffer = FastIO(file)
          self.flush = self.buffer.flush
          self.writable = self.buffer.writable
          self.write = lambda s: self.buffer.write(s.encode('ascii'))
          self.read = lambda: self.buffer.read().decode('ascii')
          self.readline = lambda: self.buffer.readline().decode('ascii')
  
  
  def inp():
      return sys.stdin.readline().rstrip()
  
  
  def mpint():
      return map(int, inp().split(' '))
  
  
  def itg():
      return int(inp())
  
  
  # ############################## import
  # 2020/11/1
  MAX_N = 3 * 10 ** 5
  MOD = 998244353
  
  MAX_N = min(MAX_N, MOD - 1)
  fac = [1] * (MAX_N + 1)
  for i in range(MAX_N):
      fac[i + 1] = fac[i] * (i + 1) % MOD
  
  A000142 = fac.__getitem__
  # ############################## main
  from heapq import nlargest
  
  
  def solve():
      n = itg()
      arr = tuple(mpint())
      b, a = nlargest(2, arr)
      if a == b:
          return A000142(n)
      if b - a != 1:
          return 0
      m = arr.count(a)
      # a, a, ..., b is not nice
      # there are 1/(m+1) are not nice
      # only m/(m+1) are nice
      return A000142(n) * m % MOD * pow(m + 1, MOD - 2, MOD) % MOD
  
  
  def main():
      # print(solve())
      for _ in range(itg()):
          print(solve())
          # solve()
          # print("YES" if solve() else "NO")
          # print("yes" if solve() else "no")
  
  
  DEBUG = 0
  URL = 'https://codeforces.com/contest/1569/problem/C'
  
  if __name__ == '__main__':
      # 0: normal, 1: runner, 2: debug, 3: interactive
      if DEBUG == 1:
          import requests
          from ACgenerator.Y_Test_Case_Runner import TestCaseRunner
  
          runner = TestCaseRunner(main, URL)
          inp = runner.input_stream
          print = runner.output_stream
          runner.checking()
      else:
          if DEBUG != 2:
              dbg = lambda *args, **kwargs: ...
              sys.stdin, sys.stdout = IOWrapper(sys.stdin), IOWrapper(sys.stdout)
          if DEBUG == 3:
              def print(*args, **kwargs):
                  _print(*args, **kwargs)
                  sys.stdout.flush()
          main()
  # Please check!
  