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 math
  import sys
  input = sys.stdin.readline
  # sys.setrecursionlimit(400000)
  def I(): return input().strip()
  def II(): return int(input().strip())
  def LI(): return [*map(int, input().strip().split())]
  # import copy, string, math, time, functools, random, fractions
  from heapq import heappush, heappop, heapify
  from bisect import bisect_left, bisect_right
  from collections import deque, defaultdict, Counter, OrderedDict
  from itertools import permutations, chain, combinations, groupby
  from operator import itemgetter
  from types import GeneratorType  # for recursion
  from typing import Iterable, TypeVar, Union  # for sorted set
  MOD = 998244353
  
  
  def modFact(n, p):
      if n >= p:
          return 0
      result = 1
      for i in range(1, n + 1):
          result = (result * i) % p
      return result
  
  
  def inv(n, p):
      return pow(n, p - 2, p)
  
  
  for _ in range(II()):
      n = II()
      a = LI()
      if a.count(max(a)) > 1:
          print(modFact(n, MOD))
          continue
      if max(a) - 1 not in a:
          print(0)
          continue
      c = a.count(max(a) - 1)
      print((modFact(n, MOD) * inv(c + 1, MOD) * c) % MOD)
  
  
  
  
  
  
  
  
  
  
  
  