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
  
  fac=[1]
  mod=998244353
  for i in range(1,5*10**5):
    val=(fac[-1]%mod*i%mod) 
    val%=mod 
    fac.append(val)
  
  
  def nCr(a,b):
    second_val=(fac[a-b]*fac[b])
    return pow(fac[a],1,mod)*pow(second_val,-1,mod)
  def main(arr):
  
  
    arr.sort()
  
    n=len(arr)
    if arr[-1]-arr[-2]>2:
      return 0 
    
    if arr[-1]-arr[-2]==0:
      return fac[n]
    
    less=0 
    max_val=arr[-1]
    for e in arr:
      if e+1==max_val:
        less+=1 
    ans=fac[n]
    for i in range(less,n):
      nums=n-i-1
      val=(nCr(n-less-1,nums)*fac[nums]*fac[i])%mod 
  
      ans-=val
    return ans%mod 
  
  t=int(input())
  ans=[]
  for i in range(t):
    n=int(input())
    arr=list(map(int,input().split()))  
    s=main(arr)
   
    ans.append(s)
  for e in ans:
    print(e)
  