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: #!/usr/bin/env python3
  import sys
  import getpass  # not available on codechef
  import math, random
  import functools, itertools, collections, heapq, bisect
  from collections import Counter, defaultdict, deque
  input = sys.stdin.readline  # to read input quickly
  
  # available on Google, AtCoder Python3, not available on Codeforces
  # import numpy as np
  # import scipy
  
  M9 = 10**9 + 7  # 998244353
  yes, no = "YES", "NO"
  # d4 = [(1,0),(0,1),(-1,0),(0,-1)]
  # d8 = [(1,0),(1,1),(0,1),(-1,1),(-1,0),(-1,-1),(0,-1),(1,-1)]
  # d6 = [(2,0),(1,1),(-1,1),(-2,0),(-1,-1),(1,-1)]  # hexagonal layout
  MAXINT = sys.maxsize
  
  # if testing locally, print to terminal with a different color
  OFFLINE_TEST = getpass.getuser() == "hkmac"
  # OFFLINE_TEST = False  # codechef does not allow getpass
  def log(*args):
      if OFFLINE_TEST:
          print('\033[36m', *args, '\033[0m', file=sys.stderr)
  
  def solve(*args):
      # screen input
      if OFFLINE_TEST:
          log("----- solving ------")
          log(*args)
          log("----- ------- ------")
      return solve_(*args)
  
  def read_matrix(rows):
      return [list(map(int,input().split())) for _ in range(rows)]
  
  def read_strings(rows):
      return [input().strip() for _ in range(rows)]
  
  def minus_one(arr):
      return [x-1 for x in arr]
  
  def minus_one_matrix(mrr):
      return [[x-1 for x in row] for row in mrr]
  
  # ---------------------------- template ends here ----------------------------
  
  
  p = 998244353
  
  qrr = [1 for _ in range(2*10**5 + 10)]
  val = 1
  for i in range(1,2*10**5 + 5):
      val = (val*i)%p
      qrr[i] = val
  
  def factorial_mod_p(n, p):
      return qrr[n]
  
  @functools.lru_cache(maxsize=2*10**5 + 10)
  def ncr_mod_p(n, r, p):
      num = factorial_mod_p(n, p)
      dem = factorial_mod_p(r, p)*factorial_mod_p(n-r, p)
      return (num * pow(dem, p-2, p))%p
  
  p = 998244353
  factorial_mod_p(2*10**5+1, p)
  
  
  def solve_(arr):
      # your solution here
      p = 998244353
  
      if len(arr) == 1:
          return 1
  
      c = Counter(arr)
      maxarr = max(c)
      if c[maxarr] > 1:
          return factorial_mod_p(len(arr), p)
  
      if c[maxarr-1] == 0:
          return 0
  
      k = c[maxarr-1]
      r = len(arr) - k - 1
  
      # (k+1)! - k!
      # r!
      # r+k+2 C k+1
  
      a = factorial_mod_p(k+1, p) - factorial_mod_p(k, p)
      b = factorial_mod_p(r, p)
      c = ncr_mod_p(r+k+1, k+1, p)
  
      log(k,r,a,b,c)
  
      return (a*b*c)%p
  
  
  # for case_num in [0]:  # no loop over test case
  # for case_num in range(100):  # if the number of test cases is specified
  for case_num in range(int(input())):
  
      # read line as an integer
      k = int(input())
  
      # read line as a string
      # srr = input().strip()
  
      # read one line and parse each word as a string
      # lst = input().split()
  
      # read one line and parse each word as an integer
      # a,b,c = list(map(int,input().split()))
      arr = list(map(int,input().split()))
      # lst = minus_one(lst)
  
      # read multiple rows
      # arr = read_strings(k)  # and return as a list of str
      # mrr = read_matrix(k)  # and return as a list of list of int
      # mrr = minus_one_matrix(mrr)
  
      res = solve(arr)  # include input here
  
      # print length if applicable
      # print(len(res))
  
      # parse result
      # res = " ".join(str(x) for x in res)
      # res = "\n".join(str(x) for x in res)
      # res = "\n".join(" ".join(str(x) for x in row) for row in res)
  
      # print result
      # print("Case #{}: {}".format(case_num+1, res))   # Google and Facebook - case number required
  
      print(res)